The SF Credit can't be directly compared to USD because the difference in economies of our country versus a downward-spiraling domain and post-apocalyptic society is enormous. Food is probably very expensive in the Starfarer world compared to ours, at least in relation to military costs. There is NO way a warship can be purchased for the same amount of money as a year's living costs of ~250 men, unless living costs are just that expensive in space. It would be like buying a military destroyer for around 50 million dollars. For comparison, an Arleigh Burke-class destroyer costs 1.8 BILLION US dollars.
So by equating Starfarer costs to US naval costs, a Gerald R. Ford supercarrier (currently in construction) costs around 9 billion USD and the Starfarer equivalent is the Astral, which is something like 225,000? credits when fully outfitted. This makes 1 Credit == 40,000 USD in military value. Consider that 1 Credit == 500 USD in civilian value, the world of Starfarer is quite a frakked-up place.
The average civilian in Starfarer probably earns 75 Credits per year in wages. Certainly not enough to buy military craft, but that means even small settlements of 100 people can pool up to buy a freighter.
Of course this is all just universe fluff-bending to fit the gameplay balance Alex came up with. A mere mental exercise rather than anything grounded in fact.