I recently loaned my Zoom lens out and it came back damaged. What would it cost to repair it? Should I just get a new lens?
Yes, you should get a new lens. The 75-300 is almost unquestionably the worst lens made by Canon, so spending any money on it is a bad investment.
What would it cost to repair it? Should I just get a new lens?
I don't know how much it'll cost to repair, but it's hard to imagine that it'll be less than the cost to replace it. Amazon has a factory-refurbished EF 75-300mm f/4-5.6 III for under $100.