There are numerous threads (including diagrams) about how to do this youself.
I can't say how much someone would charge you to fix it, but the costs of the motor is between $50 - $60 depending on manufacture and parts store. The issue can also be wiring. Check it all, replace what's questionable.
If it still goes up and down but is sluggish, FIX IT NOW before it gets stuck! Because it will.
If you are replacing the motor you have to get the passenger side motor because its the same as the one in the tailgate. It costs like 58 dollars at discount auto and is a pain in the a$$ to replace, especially if its stuck down it has to be up to replace. I'm with ironclad, replace it now.