When in doubt, just look at prices of modern day versions.
In oldschool battletech, every five US dollars (1990) was worth a C-bill
In more modern Battletech, a C-bill was worth something like 3 US Dollars (1999)
I haven't checked what it comes to in aToW, as my players haven't looked for anything that isn't already in the listed equipment section. Otherwise, ballparking based on the stuff already in there, (Especially costs for meals) works.