Useless Data, But It's Fun
So if anyone was wondering, I've crunched the numbers and come to the following conclusion. 1 credit is about 10 American cents. Besides this putting into perspective why food is say 50 credits, the actual price would be about 5 American dollars. Additionally, assuming Hadrian Sanon gives you exactly enough money to afford the fuel from Tau Ceti, to Alpha Centauri, to Mars, the price in game for 1 gallon of Helium 3 is 32.05 credits, or about $3.20/gal in USD.
Edit; I've forgotten to include my numbers for the price of He3 per gallon. Hadrian gives you about 1000 credits and says that should cover the fuel cost to Mars. She's also indicated you should clear the mission with Tuala so it should include the price of fuel from tau ceti, then to jemison, then to cydonia. My fuel consumption indicator from tau ceti to jemison said 25.49 gallons. From jemison to cydonia fuel consumption said 5.702. The total gallons needed from tc to j to c totals 31.192. Divide 1000 by 31.192 you get 32.0595024365 for the price per gallon. And with the understanding of 1 credit is 10 cents, that roughly translates to $3.20 per gallon.