Wow, you're hitting all my questions today aren't you?! Thanks.

Give me a little more info. Be very specific where it's located. How much of it? Does it extend all the way around? What surface do you see there now? How wide is the gap.

The foam runs the entire perimeter of the pool between the coping and the deck, "approx. 115 linear feet" (chemical seller's estimate). The gap is about 1/2" and the foam is about 3/8". I know when the deck was put in that the foam is about 4-6" high (whatever thickness the deck is). Currently the foam is level with the top of the deck and coping, not recessed.

Most expansion joints (I'm sure that's what this is) are a combination of closed cell foam sealed on the visible surface with caulk. I'm a little confused when you say "replaced with"

All the written quote says is "caulk the expansion joint", not what exactly they'll do. $684 sounds like a lot if all they're doing is capping off the top. Heck, even if I have to buy some specialty caulk and cut away some of the foam to get a decent layer I ought to be able to do it for a lot less. Do you think it's a DIY project?