The standard 75-300 is cheap on the used market. You can get one for ~$30-40 used in working condition, depending on where you buy it. I am making the assumption it's non-USM since the USM is generally always sold standalone due to it’s price tag. More then likely, a shop will tell you the same thing I did and will recommend a used replacement due to the cost of the cleanup and repairs. Now if this was a Canon L series it would be worth seeing if a camera shop could fix it.
Now, for a warning: Unless you're willing to write the lens off, I don't recommend removing the front glass element because of how it's mounted. I took the label off of a Canon lens I have to see if it’s easy and it isn’t. You need razor sharp hands due to how it is mounted and aligned. A paint pen and exact alignment are required. You may be okay without it professionally, but a DIY attempt is difficult without it.
Since it's such a cheap lens I wouldn't take it to a camera shop - take a shot at cleaning it yourself or replace it and sell this one for parts. Take your time, since you'll total out the lens if you break it. This video shows how to take apart the lens, but it may not be the best. However, I really can't find much that covers the non-USM lens.
Cette réponse est-elle utile ?