I agree with Cecil on avoiding galvanized, unless you have lots of bandaids!
I've built two recently, one round cage (galvanized) and one "box" cage out of plastic netting found at the hardware store. The plastic netting I used won't last as long as what Cecil refers to but it was cheap and readily available. It's been in my pond for a month now and so far holding up fine.
Size needed and goals should be determining factors for you.
So, it depends
The round cage was $100 to build, the "box" cage around $50
