Both prior responses are spot-on.
First, what they're worth depends entirely what you think they're worth. There is an entire industry (literally) built upon nonsense claims of such-and-such points being "valued" at so-and-so. That is pure, unmitigated nonsense, perpetuated by idiot bloggers and others who have a direct financial stake in getting fools to sign up for credit cards using their links (they make a lot of money off such fools, by the way). It's complete BS targeted at suckers. Don't be one. I might get very lucky and snag a nice award flight (harder and harder to do every day, you will notice) that would have cost an outrageous amount if purchased in cash. Does that mean my points are worth that ticket price? Of course not. Only an idiot (or someone very wealthy, usually a person traveling on someone else's money) would pay that much for an airline seat. I would never in a million years pay that much, but if I can snag a nice seat, that's great. It doesn't mean I would assign a "value" to my points based on what it MIGHT cost.
Second, United is in the midst of it's greatest devaluation in history. It's already started, many redemptions now cost 2X-3X what they did just a few months ago (and they force you to endure miserable routings with horrible layovers) but the fun has just started: in November they move to "dynamic pricing" where there will no longer be "saver" awards (what I call "the real price"), you are going to regularly see award costs north of half a million points. Imagine the value you will get when you cash in that stash of points!