CTM carts are excellent, sturdy Easy Entry carts and are currently priced at $415, if (and this is a BIG if) you can get hold of them. I believe Ozark and Star Lake still carry these carts, but at a markup.
CTM is located in Oregon, so their direct price plus shipping may be about the same as ordering from Ozark or Star Lake, who would be closer to you. If you are going to Nationals, major tack stores typically have booths (some may attend shows closer to you), so you might be able to order and skip the shipping costs.
The Lil Beginnings Sales Board occasionally has carts, and keep your eye on Craigslist, but be careful what you buy -- there are good carts and there are junky ones. Watch for name brands like Jerald, CTM, and the original Frontier (the real Frontier company went out of bursiness a few years back --beware the new ones that are cheap and easily bent). There are good carts to be had, but you might want to take an experienced carriage driver with you if you are at all in doubt. Very occasionally a HyperBike or a Trail Boss comes up for resale, but because they are rare, you will seldom save. Nonetheless, keep your eyes open. If there are any carriage auctions nearby, definitely check them out, but again, you may want a knowledgeable friend to lend you their experienced eyes.
I would bypass Ebay all together -- you have no chance of checking it out and a great likelihood of getting ripped off.