First, you won't be able to have the bottom of the tank drilled if it is tempered glass, which most tank are. You'll probably have to drill the back. I have heard it costing between $10-25 per hole from local glass places, if you can find one who will drill it for you. Then you have to add the cost of a bulkhead which is probably another $10-$20 depending on size. I don't know the cost of an internal acrylic overflow, or where you would get one.
A good hang-on overflow will run you $50-$75. Look for a good used one, as there are no moving parts and they don't really wear out unless they are abused and damaged.