I'm not sure this is that common, unless you have an adversarial relationship with your doctor. It's actually the opposite: in most states the Dr. must write DAW-dispense as written or 'name brand only' to get the expensive brand. Otherwise, the pharmacist can and will substitute a generic if available.

I've always found docs to be very willing to work with you on budget issues. If there's a similar med that is available in generic where their original recommendation isn't, they're happy to switch.