The estimated amount of $ spent by the USSR at the prices of 1987 - 50 billion dollars - for this war mentioned in my previous post is for comparison and to show that it doesnt make sense the soviet cold war enemy to spent 100 times much more than the Reds in this war. CIA reports also explains how they spent only 50 million $, relatively small amount of money for this ~7 years war. For the the sum U.S.A. paid i am in agreement with you, idd a few billion $ money well spent.
And I don't know how much Soviet actually spend and how their currency translates to USD at the time, minus inflation. But Soviet were going to spend a whole lot less than the US did, command economy, conscripted military, poor training. There was a reason 1/4 of all Soviet-Afghan War veterans got hepatitis in country, and over half got major viral infections, because the Soviet Union went cheap and paid for it.