On Pool Factory, I did a quick saltwater pool package for 18 foot and came up with spending right at $4000. That's with an upgraded 25 gauge liner, the biggest sand filter and a 2 speed pump, locking steps, and a hayward salt system. You can go on their site and pick a pool and customize it to see how the price is running. The cheapest salt pool on their site starts at around 1,900 for the 15 foot and 2100 for the 18 foot. How many people will use this pool? I have a 15 foot intex and it's just me and my 6 year old daughter and it feels small to me when we are both in there. If this is a temporary pool to see if you like it or to see how easy it is to take care of (and it is if you follow the methods here), then I'd suggest an intex like pool. They are cheap compared to the hard side walled pools and they are easy to put up. If you are looking now for something to last quite a few years, then the hard walled pools would be the way to go. I just wasn't sure of your goal for the pool.
Royal Swimming pools is a good site. I almost bought there because they had the pool I wanted. They have pools manufactured by the same folks who make the doughboy pools. You should probably check them out too.