Poseidon FX (I just picked one up with FSA funds) for $200 shipped. Not pretty (welds, weird chainline) and only reasonably accommodates a front brake but it's cheap, relatively light, and will likely fit 45+mm tires. (Disregard, didn't read the initial post properly)

I was looking at the salsa storm chaser, but it's a bit out of my budget. Has sliding dropouts.

What about an eccentric bottom bracket? It would help give you more choices for the frame used.