We have made comparison of growth of price of some stamps in the last 30 years! This shows that stamps from 1850 to 1920 grow more than others, that stamps from 1950 and on are growing very very slowly, that sometimes used stamps grow more than new ones .
Following you can see 6 stamps, with price mint and used in 1970 and price now, and the percentage of growth.Of course you have to count devaluation. Prices are in italian lira but at end of page you have a money converter.

Italy,Trieste A zone,1946, set of four
1970: M.5.000 - U.3.000
2000: M.125.000 - U.40.000
growth: M.2500% - U.1300%
Early italian States, Tuscany,1851, 1 stamp
1970: M.600.000 - U.175.000
2000: M.18.500.000 - U.3.500.000
growth: M.3000% - U.2000%
Italy,Republic,1945, full set
1970: M.65.000 - U.300
2000: U.1.800.000 - U.15.000
growth: M.2700% - U.5000%
Italy,Kingdom,1934,set of 2
1970: M.4.750 - U.1250
2000: M.6.000 - U.8500
growth: M.60% - U.680%
Italy, Kingdom,1867,1 stamp.
1970: M.75.000 - U.35
2000: M.3.000.000 - U.7.500
growth: M.4000% -U.21.400% !!!

Thank you for reading! D&D,Italia