The power question really is a matter of your intended purpose. Most spotting scopes have zoom lenses, rather than the fixed-focal length lenses of most binoculars. For typical nature viewingbird-watching, scenic vistas, climbers on a Yosemite wall, that sort of thinganything from 15-30x to 20-60x range should be more than adequate (15-45x is very common). The smaller and more distant the object, the more power you want. Keep in mind that higher-power scopes also will magnify any shake in the system, so get a solid tripod to go with it.
You can spend as much or as little as you want. Bushnell makes a perfectly adequate scope called the Sportview Zoom 15-45x, which sells for about $100. A step up might be the Leupold Wind River 15-45x, which offers a little brighter viewing then the Sportview, sharper optics, and better protection against bumps for about $275. Or, take a look at Bausch & Lomb's Elite 15-45x for about $750. A very nice scope, with bright viewing and excellent sharpness.
Bausch & Lomb's Master Tripod ($70) would stabilize any of these scopes nicely, as would the Slik Mini-Pro Tripod ($70).