This is how you get cactus for cheap.
First, cut off a piece of cactus. The larger the piece, the more reserve it has to use growing new roots. Prickly pear can be started from just one pad, or from a piece of multiple pads. Cholla grow easily from starts. A columnar cactus cutting should be good sized in order to successfully start.
Second, let the cactus pieces dry out in a shady place for a few days. This allows the cut to dry out and get hard (called a callus).
Third, plant your cactus cuttings with the cut side embedded in soil. You can make your own cactus soil mix. Here’s mine: I mix my hard desert clay with a third part sand.
Fourth, water your starts. You can water them about once per week. Make sure they dry out in between watering. Within a few months, a prickly pear can grow a pot full of roots and be ready to plant in the ground.