It takes a LOT of money to run an XCOM base! These are the typical monthly running costs for all items that a base can have:

  The primary means for entering and leaving the base. The Access Lift normally has Stealth technology concealed and camouflaged upper doors. (Aircraft can enter and leave via their hangars'. Normally the lift goes down around 200 feet into the base below, providing a nice, thick roof as shelter above.
  It is important to note that if the Access Lift shaft is somehow destroyed, the folk below will be trapped and the base possibly destroyed...

 Caged in thick housings, the Aliens can be held here to be studied safely and at will. Up to 10 aliens can be held in each Alien Containment unit, their offensive and defensive abilities neutralized. Excess Aliens must be destroyed or some in the containment center got rid of before more can be added.

  Each General Store can handle up to 10,000kg of XCOM personnel combat hardware. A Heavy Weapons platform (HWP) takes up 1,500kg of space (assuming sufficient ammo for one deployment, typically each load for a HWP takes up 500 kg of space).
  Aircraft ammunition are stored in the Hangars themselves, not in the General Stores!

  Aircraft need huge bays to be stored, worked on and to keep supplies for them. Each Hangar can hold just one aircraft, and with enough fuel to keep an Interceptor or Skyranger going for a couple of months of duties at least. Heavy aircraft weaponry are stored here as well, up to 30 tons of ordinance! For weights, an Avalanche Missile takes up 1 ton of space, as does its launcher, Stingray Missiles are1/3 that in size, as is a cannon and a full ammo load for it. The Advanced weapons and their ammunition take up the same space as an Avalanche missile.
  The hangars have huge hydraulic jacks that can lift the aircraft up to concealed doors leading to the outside. These massively thick and Stealth camouflaged.

  Laboratories are stocked with all manner of advanced technological aids, used in the understanding of the Aliens, their equipment and the creation of new human technologies as well. Up to 50 scientists can use each laboratory at the same time.

  These units scan the sky in a 7,000km distance, and are linked into satellite and military radars in the vicinity for precise tracking. Their chance of detecting a typical UFO is 5% per 10 minutes, but this can increase with the size of the UFO and its flight pattern. i.e. if large and slow the detection chance could be 40% per 10 minutes.

  These are designed to hold up to 50 people in relative comfort. Although a tad spartan, they do come with computers and entertainment systems for enjoyment and research. Also each unit has a small gym to keep fit in.

   These massive ground-to-air missile units are camouflaged, and open fire on any hostile aircraft out to a range of 30 miles. However they can't be reloaded except over a considerable time so when an alien craft attacks they can be used just once. However they are very powerful, they fire a swarm of missiles, so are guaranteed to hit any target, and do 2d4+1x10 hull points of damage.
  It's important to note that although they may not destroy an alien craft, they cause so much damage there's a chance some of the Aliens inside will be killed or injured (GM's decision)

  These units scan the sky in a 4,000km distance. Their chance of detecting a typical UFO is 5% per 10 minutes, but this can increase with the size of the UFO and its flight pattern. i.e. if large and slow the detection chance could be 40% per 10 minutes.

  A Work Shop can produce any of the items XCOM researches in its Laboratories, from known but not perfected technologies like laser weapons, to as yet unknown Alien devices! A Work Shop allow up to 50 engineers to work at one time, but their projects ALSO take up some space ("work shop space needed" in item entries) so this cuts down the amount of space equal to that number of personnel. 
  For example say a laser weapon required 10 units of workshop space, that would leave 40 units of space for up to 40 engineers to work.

  Based on the Blaster Launcher, these antimatter missiles can blow a small mountain apart! Against a UFO they hurl dozens of projectiles, they always hit since so many are fired,  and do 8+d4 x 10 hull points of damage out to a range of 20 miles. It takes too long for them to be reloaded to fire more than once.  
  It's important to note that although these defences may not destroy an alien craft, they cause so much damage there's a chance some of the Aliens inside will be killed or injured (GM's decision).

  This curious device projects a massive gravity wave up at attacking craft! In use this causes attacking UFOS quite some time to re-align themselves, and come on once again. This allows an XCOM base's defencive weapons to be reloaded and fired again! So Missile, Laser, Plasma and Fusion Ball Defences can fire an extra barrage before the Aliens land!

  The Aliens communicate with their ships using an other-dimensional wave of highly unusual properties. When XCOM can decode this, it shows exactly what a UFO's type is, along with the race manning it, and their current task!
  Hyper Wave Decoders have a range of 8,000km and are instantly and 100% effective!

  Using the base's generator, laser Defences fire a storm of light that can melt through 12' of steel! When used against a UFO it's guaranteed to hit, due to the amount of attacks.
  Laser Defences inflict 3+1d3 x 10 Hull Points of damage on attackers out to 15 miles
. It's important to note that although these defences may not destroy an alien craft, they cause so much damage there's a chance some of the Aliens inside will be killed or injured (GM's decision)

  An incredibly complex blending of Alein technologies and human leectronic engineering, a Mind Shield helps blanket out the mental process of base personnel from snooping aliens, who use psionics to help spot concealed bases. 
  In practice, the chances of an XCOM base fitted with a Mind Shield being detected by Aliens is reduced by 50%!

   Hurling superheated bolts of energy into the sky, Plasma Defence units can smash most craft out of the sky with great ease.
   Plasma Defence batteries inflict 6+1d3 x 10 Hull Points of damage on attacking craft. As usual with base defence installations, they can usually fire only once before the enemy is close enough to invade.
  It's important to note that although these defences may not destroy an alien craft, they cause so much damage there's a chance some of the Aliens inside will be killed or injured (GM's decision)

  This base unit is vital, as it allows the training of XCOM personnel in psionic warfare! (see Combat for details)  Up to 10 soldiers can use the Psionic Laboratory each month