My suggestion would be to look at costs of Easyjet, who have a regular service from Glasgow to Berlin, and then rental cars (useful whilst you are there), or if a lot a small minibus. Then compare that to a charter.
If you are not used to charter you may be surprised at the overall substantial cost for an oddball flight, not just for the aircraft but also crew accommodation, charges for empty leg flying, etc - notwithstanding the enthusiasm of some here who actually work for such organisations !