OK...I'm an Assos pundit...
I can't help but mention that a quick search finds that the Descente and Castelli bibs mentioned above cost as much as the low-midrange Assos...That being the case, if you're going to spend the money, go Assos.
However, what the OP asked for was best bibs that don't cost as much as Assos.
The best bibs I ever owned that cost less than Assos were PI Microsensor's, but once I spent the extra $10-20 to get Assos FI.Uno's, I relegated my Micrsensors to "emergency, I forgot to wash my Uno's" status.
I recently bought some Castelli's with the Kiss bib trying to be cheap, and boy was I sorry.