Depends on what all they're doing. If it's just lube/adjust stuff, then I'd say less than $40...if it's replacing stuff like tires/tubes/chain along with repacking wheel bearings and similar labor-intensive stuff then anywhere up to $100 or so.
Honestly, you can do most of this yourself if you've got reasonable mechanical aptitude and patience. Most bike shops will have good books on DIY maintenance and tools aren't astronomically expensive for the most part. You can outfit a basic bike toolbox for about what one major overhaul would cost you.