I bought a 16gb kingston class 4 on amazon, it was around 45$, and they had a promo that if you signed up for an amazon credit card you get 30$ off your first bill. so bought it and paid it off for 15$.
Honestly, with Froyo coming soon and apps2sd, Id really reccomend investing in a good class 4 or 6. The faster the speed of the card, the faster your phone will be able to pull and push data to it.
Another note, my class 4 can move large files at about 8-9 mb/s, but if there are many small files it will only run about 3 mb/s, so Id really recommend a reputable brand. That is not to say the kingston doesnt work good, just to say one of those cheapo 30$ cards advertising class 6 will probably run much slower and break apart quickly