If you know what you're doing, do it yourself and charge 20% lower than the pros. You will make decent $$$, but be prepared for some nastyness(unless it's a new building). You can also get a couple of cheap labor folks to do the runs for you. I know a few of them that always hang around the local DMV...
If you have not done one before, either do this one to gain experience, or hire a pro. My recommendation is to hire a pro, because they know how to run them correct and make sure to label and test properly. The better ones will have all the lines punched down to a patch panel with cable management on a 2-post rack.
how many lines? since you said mid sized, I'm assuming 100+ lines? If that's the case, definitely hire a pro...
I've seen them as low as $50/line to $250/line depending on distance and how messy the space above the tiles is...