Call up Cogent Communications. Ask them where the nearest carrier-neutral data centre is where they could give you a 100Mb transit connection and some simple IPv4 service (some small amount of PA space and a gateway), and how much it would cost you to use it all. That's roughly 25TB traffic, and about the smallest sensible amount of "wholesale" bandwidth you can purchase. Cogent are going to be quite cheap, and you'll be able to use the whole pipe. I'd imagine it'd be in the order of $500-1000 per month, so around 2-4c per gigabyte?
Then call that data centre and ask how for much they could co-locate a cheap 2U box (or if they have a customer who would rent you a small amount of rack space). Ask how much a cable run to Cogent would be.
Add it all up, and that's about as cheap as you can get it, at least starting from scratch. Even if you don't do this yet, you'll know how much other hosting companies are marking up what they sell. For comparison call Level3 for some "quality" bandwidth (you might need to ask for a reseller if you "only" want 100Mb). Or see how you feel about the costs of a second connection, BGP, ARIN membership and all that madness. You'll soon be your own ISP :-)