Buying the part yourself and installing it can save a lot of money! Search for a great deal online and you're more than likely going to find it. It's always good practice to check out the prices for a spare part for your Canon EF 300mm f 4L IS USM from several different places. That way you can be sure you're getting the best deal. There are usually more than one company producing the Canon l series telephoto prime lens for canon eos slr cameras part that you're looking for, especially aftermarket-type spare parts. Find the right part and fix it right!