Pondering a longer lens

I'm getting fed up with how difficult it is to shoot birds, especially smaller ones with "just" a 70-300. So I'm pondering an addition. Problem is, as soon as you get beyond 300mm things get expensive really fast.

Sigma 50-500mm F4.5-6.3 APO DG OS HSM
Sigma 50-500mm F4-6.3 EX DG HSM
Sigma 150-500mm F5-6.3 APO DG OS HSM
Nikon AF 300 f4 IF-ED
Nikon AF-S 300 f4 D IF-ED
Tamron SP AF200-500mm F/5-6.3 Di LD (IF)
Nikon 400mm f3.5
Sigma 400mm f5.6
Yashma 300mm f2.8

Is the list so far. A few like the new Sigma 50-500 and the Nikon af-s 300mm are way too expensive. Ah, what to do?