I'd say people are more likely to let go of their bike for a lesser price during the winter if you can wait. I purchased my 09' 250 this past winter and got a steal of a deal. The only issue is you can't test ride if or have the seller test ride it on snow/salt covered roads. They question is how long you can wait.
If you're buying this summer, depending on the year, mileage and condition I'd say anywhere between $3000-3500 would be a great deal.
Edit: To more specifically answer your question..
Buying over THIS summer: $3000-3500 is a good deal.
This winter: $2700-3300 is a good deal.
Summer 2014 (with the possibility of used 300's on the market): who knows???