The AMA discount is 20-25% off Garmin's retail prices which make them competitive to street pricing.
Here's a deal if you have some time:
If you have an eBay account and Paypal account, go to www.live.com
, create a MS account and search for a gps (or anything else that you're in the market for). Click on the link to eBay and you have 60 mins to find a buy-it-now item and get 25% off the best BIN price on eBay.
Read the fine print but basically you have up to 12 purchases to get 25% off (does NOT include vehicles). There is a 60 day wait for your refund.
There are deals for other sites for less off but still worth looking into.