When I put it on reverse it doesn't do anything
My car has an automatic transmission.
And if you could get an estimate on the price to get it fixed
There are a few things that can cause this. Check the transmission fluid level to see if it is low. Sometimes low transmission fluid levels will prevent the transmission from shifting into gear. If you still have forward gears, but just no reverse, it could be a problem with the reverse clutch or possibly with the valve body, located within the transmission. I suggest that you contact a reputable local transmission shop for a full diagnose your transmission.
However, If you’d like a second opinion on this, consider YourMechanic, as a certified technician can service your transmission fluid if necessary and diagnose your shifting issue firsthand to give you a quote.