most of the time, it's $150, and the sights are not new. So it's more like $30 per year, because replacing them is a sort of semi-custom job. Still, it's chump change, well worth the expense.
I do my own, so it costs me what the sights cost, which for the last couple of sets, was $68 on sale. They were new too, and were dated the same year as they were installed.
Replacing them is easy enough, and so far, I usually recover half the cost of a new set, selling the old ones on EBay. That alone had paid for the sight tools.
Of the 6 sets of used sights Ive replaced, I have yet to replace a set that were totally dead, and most were beyond the 12 year warranty date when they were replaced. Even then, while not as bright as new, they still glowed, and were visible in the dark.