It used to be a maximum of $500, though I don't know anyone who was asked to go that high. Most anyone I know that was asked to pay one was around $150.
Back in 2002 they asked me for an $800 deposit. I was 18,I guess a lot of young'uns at the time had screwed them over or something. Needless to say I stuck with prepaid for a while.
I work as an Indirect selling both AT&T and Verizon. I've actually seen AT&T deposits up to $750 and yes that was for ONE line. Blew me away. Most of the ones I have seen were $350.