As Daryk stated, the game has always worked on the idea that 1 C-bill was valued at $5 U.S. Dollars.
This tends to breakdown with some of the more low cost mundane items, but is close enough for government work. :thumbsup:
All the Era books have the breakdowns for the house per time period after that.
The general idea seems to be that money is based on the C-bill/SL dollar standard from there.
If you are looking for a direct conversion, I would find an item that is as close as possible to something we have today and compare prices.