I gather that pins at WDW run $7-15 depending on the pin. That's too rich for my blood, so I'm getting mine off eBay.
Now, they are considerably cheaper on eBay, but a lot of them are apparently "scrappers" or unauthorized, inexpensive reproductions of real Disney pins. It's hard to find inexpensive, authorized pins, but not impossible.
There is a thread on the Collectors board here about pins, and I've managed to find a number of sellers on eBay that look legit. Problem is, their pins are still on the expensive side. I was warned to look at the sellers and see how many pins they are selling. Those that are selling a lot at $1-2 each are probably selling scrappers. Look for people with smaller lots that indicate they bought or traded for their pins. Or look for pins that are still on cards.
I just won an auction of legit Star Wars pins, 4 of them, still on cards, for around $11, including shipping. I've found another auction that I'm pretty confident about (seller was listed by a member as being ok). Those pins are going to run around $2.10 each with shipping.