Assuming that your financial situation can withstand it, I'd say yes to the standing desk. I'm honestly not sure what my desk at work cost (company purchase), so I can't give a very precise cost/benefit analysis or tell you what's a good price point, but I do absolutely cherish my standing desk.
Unresearched opinions and anecdotes:
* It wasn't so much painful but I was aware of my feet for the first week. Don't let yourself get locked into one stance; shift your weight slightly or walk a few paces, and you'll relieve that sensation. Now I shift my weight without thinking (and walking around is a good idea anyway). Fatigue mats or yoga mats are what some people use.
* Your back won't necessarily thank you right away. You may find that you're engaging small muscles that have been neglected. Keep a good posture (head upright, shoulders not rolled forward, core muscles lightly engaged) and you'll get stronger for it.
* This should go without saying, but what you do in your non-desk time affects how you feel. I am a runner, I bike or walk to work, I eat healthy, and I sleep on a fairly nice mattress. The adjustment may be harder -- and the benefits smaller -- for people who don't have a well-rounded health profile.
* It sounds like this is already on your radar, but standing desks aren't an all-or-nothing proposition. I usually sit once around lunchtime and sometimes again before I leave work, for a half-hour each.
* Use some type of guide to calibrate your standing desk -- I think http://www.ergotron.com/tabid/305/language/en-US/default.asp... is a decent one. Some people find themselves standing too close to their desks when standing (used to hunching forward into the monitor?); this can be somewhat mitigated by ensuring that your keyboard and mouse are in a location that reinforces the proper distance.