There is a tarnish to the eye of the lens Sigma Zoom 20-200mm
From your picture it is impossible to tell if the blemish is on the rear element or internal. The first thing I would do is clean the external lens surfaces then take some pictures. You may find that the blemish has no effect on picture quality. If that is the case, I would do nothing.
If it does affect your photographs, you will need to take it to a camera repair man. What ever you do, do not attempt to disassemble the lens yourself. You will not be able to get it back together and only add to the cost of repair. As far as cost, that is up to the camera repair man you choose. Without looking at the lens, he will not be able to give you an estimate. Check the cost of repair against the replacement value of the lens on E-bay and then make your decision.