Depends on what you consider a decent hot tub. You can get something basic like a Nordic Retreat or a Freeflow Monterey for somewhere around $5k I believe and the quality will be acceptable, but the hot tub will be basic. You can step up to the $7k pricepoint where you can get into a entry level hot tub from Hot Spring, Jacuzzi, Caldera, Marquis, Bullfrog, Sundance, Artesian, Arctic, and D1 and your going to have a more aesthetically pleasing spa with more therapy, lights, filtration, etc.
Used is difficult because brand and condition are very important. If you can't see a used hot tub filled, heated, and running, then its basically worth nothing.
I would start by going on the webites of the brands I listed above and use their dealer locators to see who you have locally and start there. If you don't have the option available locally, then its not worth considering. Find out if you can run 220V or if you are limited to a 115V option. Also look into your local retailers. The dealer is as important as the brand since they are the ones who should be taking care of your service needs after the sale.