Budget depends on what you are going to do. It's like any hobby. You can spend as much or as little as you want. There is the Budget Bus on youtube:
Don't think he spent too much, then there is the good news bus:
who looks like he spent a lot more.
I plan to put about 15k on top of purchase over a year and a half. But that includes a generator, paint, flooring seats, electrical, roof patio, shower bathroom, etc. If you want to build a primitive cabin on wheels I'm sure you could get away for 5 k depending on how nice you want to make it. Also if you do a lot of the work yourself you'll save a lot of money.