Upgrade cost is about right, the main prob is that to run decent science it's difficult to put away the cash for upgrades- & the AI has cheap upgrades & rarely has enough cash put by to pay a decent amount for techs or for resources
Slightly off topic, I don't know if it's possible to mod this, but I think a solution may be to convert trade income from commerce into gold, which should:
i) free up AI gold to enable realistic tech & resource trades
ii) slow down scientific research.. & thus enforce more tech trading
iii) make gold-earning buildings such as the market, grocer, bank & Wall Street more useful- often (on noble) with a religion or two it is possible to run close to 100% research. Often, banks are built just to get the benefit from Wall Street in the main religious centre, but most of these are then useless once research gets cranked up to 90% or 100%
Just my opinion... views anyone? (or even better, ideas as to how this could be modded from anyone much cleverer than I!)