It's outrageous how much they (all rental companies) charge for car seats.

They are generally free to check, however so that may be a better way to go.

I've been going through this hassle trying to plan for my family's trip this summer. Unfortunately we're going to Europe so our Notth American car seats aren't certified, therefore we can't use them there.

Our solution is to can into the city (in France and the UK kids aren't required to have a car seat in the cab) and then have a family member bring us booster seats the next day when we pick up the car. Of course, because it's booster seats those are cheap enough to buy new (we can get them for less than 2 days worth of rental on them)