Hi Maria
I don't know of anyone who does 'budget' conversions, but firms like New Dawn, Celtic Motorhomes, Clearcut, and 321 Away, are well thought of, along with others I haven't thought of.
As for price, it would depend exactly what you require, but do bear in mind an unconverted Bongo with a loo and brew kit is plenty adequate for camping.
Not a brilliant answer, but hope it helps a bit.
