How much should I expect a plumber to charge me to unclog my bathtub?

I live in a historic house in Savannah. The bathtub has been clogged since we moved in and I've finally had enough. The water doesn't drain at all during a shower and takes about 20 minutes to drain onced we've stepped out. I want to hire a plumber to fix the problem but I don't even have a rough guess on how much that will cost me. Any idea? Thanks!