You will have to put in a lot of your time in your business, and the return is good. The fact is that dry cleaning store only do small alterations i.e. heming the pants. If you want to open an alterration shop, you may want to check out the Stitch It. http://www.stitchit.com/index.htm
to see what kind of traffic they are having, and the rate they charged on various kind of services they provide.
Opening one in shopping mall will be ideal. Lot of clothing stores don't provide tailoring services, therefore, you will have enough clients; and because it is a service business, you don't need to open in a prime spot of the mall, people need your service will look up the mall directory to find you.