I've got my BD in a 4X2 75 gallon and he loves it.
Got it cheap from the pet store since it has a slight chip in one corner and is no good as a fish tank but great as a terrarium.
If you're looking for something inexpensive you can consider either building your own tank or a reptarium.. they're fairly cheap, easy to assemble and disassemble for transportation, although i have heard rumors that htey can filter out some of the UV from lights or sun.
I'd recommend getting a reptarium to start since they are cheap and don't necessarily filter out all the UV rays...(at least as an interim cage until you can afford something bigger.
I can't remember how old you said your dragon is but you may want to consider getting a smaller one anyways if the dragon is still little as he'll feel more comfortable and secure.
if building your own tank do some research on it since fumes from certain finishes and chemicals can be bad for dragons. I've noticed there is usually some sort of conversation going on in the Iguana forum on kingsnake since alot of iguana owners have to uild their own once Igs get bigger.
hope this helps
Derek