It might be worth it to look at some Cabelas tents. I've used a Trekker Dome for years and it's never failed me. You can sometimes get great deals by looking in the bargain cave at Cabelas stores. I bought the one I have in Hamburg, PA for like $45 brand new but opened. It's worth a shot to call the Sydney, NE store and see what they have for tents in the bargain cave. I've slept in it down to 33 degrees just this year. I did have my 20 degree bag, so that helped. When I opened the door in the morning, I felt the cold air rush in (or the heat rush out) so I know it holds heat with the rain fly on. And, it keeps the rain out.
$500-600 for a ground tent is too much to pay IMO. Although, I just camp with the kids or my dog, and if it's that cold, we'll sleep in the truck.