The sub-100 dollar tents are cheaply made, with heavy materials. The poles are heavy, the fabric is heavy, water will come in the seam on the floor, the side, the fly. Most often these are PU coated nylon - PU coatings will degrade and flake away and cannot be renewed, leaving you with a useless tent. Many have partial flys which do nothing in wind driven precipitation. They are often poorly engineered and collect condensation like they were made to do it.

The more expensive tents are made with better materials, have been better engineered to provide ventilation that fights condensation on the interior without compromising rainproofness, and the poles are not so cheaply made they bend or fall apart within a few uses of the tent. And they will be much, much lighter. You get what you pay for, generally. Some of the $120-160 tents like the Kelty 2 person models are decently made but heavy; spend a little more and knock a pound or two off, you have the REI brand Half Dome or one of the Sierra Designs tents. Spend more yet and you get something like the MSR Carbon Reflex 2 at 3 lbs 4 oz (great weight and weather worthy to boot).

the more often you backpack, the more every pound matters. If you go once in a while and are on a budget, look at REI, Kelty, Sierra Designs, and similar priced models. Good value for the money.