It always stings when you buy a device just before the price drops. If you happened to pick up a Nexus 7 directly from ASUS or Google, though, you may be in for some good news. Two separate partial refund systems are in place to compensate you. If you're in Europe and you purchased the slate directly from ASUS prior to October 29th, 2012, you can register for a €30 (or ₤25 for UK owners) voucher that's good for use in the company's online store. The offer doesn't seem to specify which versions qualify, so it may be all of them.
Over on Google's side of the fence, there is actually a persistent Price Protection policy that has kicked into effect on the 16GB variant of the Nexus 7. If you purchased the device within 15 days of the price drop (so, on or after October 14th), then you're eligible to receive the difference in price back (about $50).