They're all OK lenses. The 75-300 is a bit soft at 300mm, but otherwise OK. The 50 is a good lens, though its all plastic construction leaves something to be desired. The 20-35/3.5-4.5 is an older lens, but still has ring USM and it has full frame coverage. I still have my 20-35/3.4-4.5, though I don't often since I also have other lenses that cover that range. However it's still my widest lens with full frame coverage.
The best place to find out what current prices are is to look on
eBay at completed auctions. That will tell you what lenses are actually selling for, not what the "asking" prices are.
I'd say maybe $200 for the 20-35, $75 for the 50/1.8 II and $120 for the 75-300 (assuming it's not the IS version), so $400 for all three would be about right. If it's the IS version of the 75-300, that would add at least another $100 to the price.