Really depends on the amount of data on your site; I had to do a cleanup of a really nasty hack on a client site where I had to work with his webhost directly to clean up the mess and it took about 12 hours to accomplish (though his issues extended far beyond his forum just being hacked).
My general baseline minimum as far as a quote would go is ~3 hours @ $120/hr; which would include comparing ubb files for any "new" files, looking at all files newer than a couple of weeks, going through every line of code on your site and looking at post requests, etc.
