You can program the remote yourself (do a search for threads on howto), but not the key itself. I know because I lost a key and a remote and had gone through the process of replacing them. My local dealer wanted an arm and a leg for them, but Tim will replace and program the key for $25, and the remote for $85. You'll need to send in a key for the reprogramming; I sent my valet key.
Someone mentioned that any Acura or Honda remote can be programmed to work on MDX, though I haven't try that myself. If so, you can probably get one from EBay real cheap.