Cost-wise, repairing it doesn't make sense unless you somehow find a nearly free nonfunctional camera to cannibalize for parts, in which case you have nothing to lose.
But realistically, using U.S. prices:
The mirror box assembly costs about $80, not including any labor to install it. I would get the whole assembly, because you never know what other damage might be present.
You will probably spend another $10–$20 on a new focus sensor, unless that's included. (I don't think it is.)
The problem is, if you're unlucky, you may replace all of those pieces only to discover that the body itself has some subtle damage that causes these parts to not remain properly oriented with respect to one another. At that point, you've wasted most of the cost of a replacement camera ($160 for that model used) and you have nothing to show for it.
So it seems silly to spend well over half the replacement cost just to repair something. Your odds of success would have to be almost 100% for that to make sense, and that's really not the case for something like this.
In theory, a repair shop could tell you if it makes sense to repair it, but in practice, they'll charge about as much just to look at it as it would cost to replace the camera.
So my advice would be to just shoot in view mode mode until the mirror completely stops working, and then find yourself a used replacement. Live view mode photos should be unaffected, because that mode isn't dependent on the standalone focus sensor inside the viewfinder or the mirror (so long as it stays up and out of the way).
If live view mode is affected, then the problem is probably a decentered lens rather than a focus sensor or mirror problem, and that's a whole different problem.
