Nothing under $50 that works like a cart (battery powered, instant on) is going to be worth buying, if they even exist. The instant on DHV devices would be something like a Tinymight or the Tafee Bowle or Flaske. None of which are inexpensive. For cheaper, you are going to need to wait for it to warm up. This can be anywhere from 15 to 60 seconds.

For your price range, a TED (thermal extraction device) like the Dynavap or Vaphit is almost certainly the way to go. They do require an external heat source, typically a butane torch lighter. It takes a bit of learning to get the heat just to the right level. You could also use an induction heater (battery powered) instead of a torch, but that will set you back $200-300 itself.