can't get back wheel off because has rusted on. Cost to replace?

OR , depending on location , driving conditions and level of common sense--- Put all lug nuts on , but go hand-tight , then back off one turn. Move vehicle back-and-forth in driveway or safe area in road , you might notice when it comes loose........

Should be able to get it off without resorting to that. Do you have your own jack stands, or are you just using the factory emergency jack? DO NOT try this with the emergency jack that comes with the car. If the car is secured on jack stands, or with the car on a proper four post lift, you should be able to break the wheel loose from the hub with a few smacks to the tire face from a rubber mallet. It is not necessary to cut off an axle to deal with a stuck wheel, that would be egregiously expensive and more involved than it sounds.

Also - liberally spraying wheel at lug studs and back side with a penetrate such as PB B'laster or WD-40 will help loosen rust and aid in wheel removal... It's got to come off without cutting it off! .. If vehicle is still drivable, consider visiting repair shop... Hint - *Replace and tighten lug nuts* first!