So far US resorts have not surfaced on Priceline but they HAVE been on Hotwire, but for still more than $100.
The $99 AP or entertainment rate shows up every now and then but very rarely. You'd have to be very persistent and call regularly and ask if they've come through. This can sometimes change week by week.
I just want to add that the rates may be higher but the UE perk and the access to the parks are well worth the rate. I've done both and I'm the CHEAPEST human being on earth, but there is nothing like Universal onsite with the FOTL perk, even if you only book one or two days it's worth it.