If cheap surrounds are required, there's always the
Fluance AVBP2. I'm happy with mine as rears. They're pretty cheap, and they sound about as good as anything would playing behind my head.
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
It'd be nice if they were ported rather than sealed and could reach a little lower, and as such I would probably prefer the XLBP model. Still, I have absolutely no complaints about their clarity or timbre.
I think the Bostons for LCR would be an excellent choice, and the Dayton sub is going to be hard to beat within your budget. You're on a good path here.