kb,
Realizing that you are going to be dropping from the current 400HP to 250-300HP if you go with a single, your options are pretty limited from a "cheap" standpoint. The current 300 HP singles, whether 4S or 2, are all relatively new, and thus will carry a heftier price tag. Suzuki 300 (4S), 300 Vrod (4S), 300XS (2S), 300 Etec (2S) and 300 Yam (4S) are your offerings, with the Yam being the new kid on the block.
If you choose to go with twin 150s, your choices are quite a bit more varied, and in your price range. Hell, I am listing for a friend a pair of Yamaha 200 HPDIs with all rigging, binnacle, gauges, for $9K OBO, and I know he'd take less...
Was that a shameless plug, CB?