Is it expensive to switch from an 80% to a 90% efficient furnace, to comply with the new mandate?
PVC is relatively inexpensive so the main cost is installing it. If your furnace is near an exterior wall, they'll simply make a hole in that wall and run the lines directly out. If your furnace isn't close to an exterior wall, then you need to run the lines through the ceiling, preferably in the direction of the joists, until you reach an exterior wall.

If that ceiling is already finished with drywall and has other utilities running through the space, then the expense will be to open the ceiling and get the vents properly routed. Drywall repair can be a DIY job, though mudding the joints tends to be an art if you want it to look perfect. And most HVAC installers would likely outsource that job to another contractor. The second contractor is where the price on the job can start to jump.

That said, the added cost is only in that specific scenario, which is a minority of the installs. And you'll recoup that cost over time with a more efficient HVAC. Therefore, I'd avoid prematurely replacing a furnace since that will be the most expensive option of all. And I'd also recommend against using contractors that are sending out fliers or knocking on doors to get business. The good ones don't have to look for work, they've got a waiting list for their services.