Mine is two 4x4's with half inch holes drilled through and a 3/8" rod shoved through one hole through a "core" in the middle and out the hole on the other one creating a stand. A core being one of the cardboard tubes that vinyl/banner/etc is rolled up on and what you're left with after you yell out various swear words and your job cancels because you thought there was enough. I tape the media to it when it starts getting long and roll it up when needed by just turning the core. To unroll, I do similar to a tape device, grab another core and start rolling onto that while unrolling from my take-up, remove it, then roll it on a flat surface to cure. Not the best of devices but it works for me.
I've seen some folks make media take-ups/catches on here using banner material that's kind of like a hamper. Seems to me I also saw someone on here who built a system using a stepper motor and either a light sensitivity switch or an IR based mechanism to start feeding the material and it also had a heating system as well if I recall. So yes, you can build your own, and its complexity can vary from simple mechanics to being fairly adept and electronic.
And as to the question of cost, mine was dirt cheap for me to make, but I know the professional systems can run quite heavy, think my dealer wanted like $2000 for one.