It may get expensive though.
Only if the exchange rate is wrong. If he gets the exchange rate correct (difficult to do in a centralized manner) then I think it should not get expensive. He would be able to sell one for dollars or other currency and then buy the other. He may get a lot of volume. But without actually creating such an exchange, there really is no way to know.
In a way, sites like coinabul are an over-the-counter gold/bitcoin exchange. I don't think a gold/bitcoin exchange similar to Mt.Gox currently exists, but I think it would be a good idea to eventually have them in addition to the OTC variety exchanges. Precious metals and bitcoin complement each other quite well, in that all are very difficult to counterfit.
Without destroying what it is that makes bitcoin work, exchanges between gold and bitcoin are the only way to link the two currencies.