As you noted, this lens isn't that expensive new and certainly not second hand and because of that trying to repair it yourself has a very high probability of costing more than the lens is worth in terms of replacement cost. The issue could be as simple as a broken trace on a flex cable inside the lens or a failure on the control board inside the lens and without service documentation and some basic test gear, you won't know until you start throwing parts at it and that will get expensive in a hurry. And if you don't have experience working on precision assemblies that like to stay clean and have a lot of fragile connections a camera lens is not a good learning experience. Plus your only inexpensive source of parts for this lens will be from cannibalized lens on ebay or elsewhere and those come with a very high probability that the component part you buy will also be defective regardless of seller claims and when you get it you won't know if the "new" part is also bad or if the actual problem lies elsewhere.
I just went down this road with a Canon EF-50 F1.4 that is the only Canon lens that has ever caused problems for me. It was repaired once under warranty and again on my dime a few years later and now it again works fine but only in manual focus. The issue with the 50MM F1.4 AF is known but it isn't worth my time to repair what is an OK but not exceptional lens and I acquired a barely used Sigma 35MM F1.4 art series lens from a friend who is changing camera bodies which made more sense for me than buying another Canon 50MM F1.4 given the Sigma appears to be a very good design. I doubt if it is worth the expense to have Canon repair your lens but if you want it done right, that is the correct option. Otherwise replace it and save yourself time, trouble, and probably expense.
I repair a lot of complex electronic and electro-mechanical stuff down to the component level but only when it makes sense to do so (either clear cost differential or because there is no other practical option) and I have the array of test equipment and tools to do this sort of work. But for a common piece of glass that is available used working in good condition for "cheap" it would be an easy choice for me to just replace it.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
