As Ricksaid, many GM cars use Dex-Cool. Whether your Prizm uses it, I don’t know. I did look up a '99 Prizm, and my info showed it used non-Dex-Cool antifreeze and had a replacement interval of 30,000 miles.
Dex-Cool is noticeably more expensive than traditional antifreeze. In theory it’s cheaper to use because of the extended service intervals. It’s not as much more expensive as the difference between $50 and $80 suggests, but that price difference is almost certainly because the $50 is a lowball special price designed to be competitive, whereas the $80 is a “regular” price.
It has nothing to do with high mileage engines.
It’s not necessarily a scam in the way KP suggests. There are three possibilities: a) your car actually calls for Dex-Cool; b) it doesn’t but the dipwads at this shop think it does; and c) it doesn’t and they know it. C is obviously a scam, but B is a more common scenario. Unfortunately, lots of people who earn their living doing auto repair are poorly trained and not fully competent. They cost customers extra money not by nefarious design, but by exerting their ignorance with an air of authority.
The antifreeze protection (temperature range) of Dex–Cool is the same as traditional antifreeze. They’re both ethylene glycol. The difference is the additive package for corrosion resistance.
It is generally not a problem to switch from Dex-Cool to traditional or vice versa. Many GM cars have been changed from the former to the latter because of the problems Rick mentioned. GM cautions that the system needs 100% Dex-Cool in order to have the extended life claimed.
Sometimes, however, it is a problem to switch from the type of antifreeze used at the factory. It’s not common, but there have been cases of cars that develop severe corrosion problems from doing this. I’ve read about it in some industry sources and have no doubt of it, but I’m sorry I don’t have a link or know enough about it to explain it fully. It essentially has to do with the different effects of the various additives used and how the metal in the system reacts to them. The additives bond to the metal and are not removed by flushing. Getting different additives in there, often in conjunction with a new metal part (e.g. water pump) that doesn’t have the same stuff bonded to it can kick off a chemical chain reaction that starts eating pumps, heads, blocks, radiators, etc. In light of this, it’s not unreasonable for a conscientious shop to insist on using the factory-type antifreeze.
The best advice I can give is to stick with the type of antifreeze used at the factory, and flush it at least as often as recommended. In my experience, more often than not the acidity level (corrosiveness) of the coolant reaches an undesirable level by or before the maintenance interval.