Ok so let's cut to the chase, I purchased a 70-200 f2.8L non-IS lens off of ebay a few weeks ago for $426 shipped. Still focused, but somebody had scratched the living BAJEEBUS out of the front lens element, and then tried to polish the scratches out!! Needless to say, the element was stuffed, but besides that the lens was fine. So I sent it off to Canon for a new element, clean, test and calibration....
Stupidly, I didn't write on my service note 'please quote' and instead wrote 'please replace...'
So they went ahead and fixed it all and the bill? $766!!
So a lesson to be learned here, I STRONGLY recommend you buy a good quality UV filter because no matter how expensive it is, it'll pretty much always be cheaper than replacing the front glass
ps: i'm still glad I got it done, cos it all works out to be about the same as a secondhand lens that HASN'T been so thoroughly checked over, so i'm still 1-up...if only just.