I sold probably 3/4 of my collection between 2009 and 2010 just due to desperate circumstances. Not a pleasant experience and I'm still trying to trace down some CDs that I regret parting with. Fortunately, not that many of them were that rare and there were plenty I tossed that weren't missed at all.
At this point, I'll toss them here and there. The litmus test is usually picking something up with great enthusiasm, spinning it once, and then seeing it on my "to listen" shelf forlorn and neglected after six months or more. Hey, I'm all for collecting, but why have an album I'll never spin taking up space a new favorite could occupy?
Anymore, my first check is to enter the barcode number into Amazon to see if I can list it there with any expectation of an ok return or even just breaking even. I get lucky from time to time there. After that, I'll just leave a stack of "don't bother" cds and let it build up until I have enough to be worth a CD's price in store credit or so.
Outside of box sets, I don't think I've ever spent more than $20 on an individual CD. I might have gone $25 for a double disc, but shopping around usually makes that unnecessary. The Budgie discography reissues have me tempted to violate that limit though - all single discs running around $25.
I did manage to resell a few Pink Floyd solo albums for between $60 and $80 a few years back though. Considering they were kinda "meh" anyway, talk about free money!