Here's a kind of sort of complet-ish improved nations mod.
Basic formula I used to determine pricing was:
All nations contribute a flat fee of 75 to the XCOM project.
Then they're assessed a basic GDP contribution fee of their GDP divided by 125. E.g. a nation with $7,751 per capita GDP would have to pay 62.01.
Next up is population contribution fee; population divided by 5,000,000. A nation with 1.2 billion people would then have to pay approximately 240.
Finally there's special fees:
NUCLEAR POWER: 135 (Known nuclear powers like US, UK, France, Israel, India, Pakistan, China, Russia)
SHARED NUCLEAR POWER: 62.5 (People who share nukes under dual NATO control -- Germany, Netherlands, Belgium, Italy, Greece, Turkey)
It's important to note that 1999 was a long time ago regarding economic growth -- Russia was still in the dumps following the breakup of the USSR; along with many former WARPAC states -- Germany was still swallowing up Eastern Germany, etc.
