TPMs have come down in price - and they only last between 4 and 7 years before the batteries tire (the outer end of that range being under "gentle" conditions I imagine). If you're still on the originals, they have done reasonably well. You should be looking at $300 installed - less if you get it done while you're changing tires. Dealership charges will be more. Aftermarket versions are available on eBay (set of 4 for ~$80) but, once you factor installation into it, you're really not saving much and I don't know how good they are. I would avoid used ones for similar reasons.
A good rule of thumb is to swap them out - whether they're working or not - at the time of any tire change between year 4 and 6 after first in-service.