My mobile was US $40, but it is now $30, and it can run one of those(the J2ME version). I also have no contract or monthly fee at all (Net10).
Good for you :-P
But I already have a mobile phone. It's simple, and does everything I need (just phone calls, text messages - and being so simple, it has long battery life). It was free from my service provider, who give me more free airtime and text messages than I can ever use, for peanuts - because it's part of a cheap bundle deal for mobile, landline, cable TV and cable internet.
If I switch to a new (smart)phone my bundle is messed up and I will pay more. If I buy a smartphone separately instead - I will have an extra phone that is useless to me except for one purpose: GW2 authentication, and likely cost monthly too.
I live in the UK, so whatever cheap deals you get in America are unfortunately no use to me.
What I really want is a hardware authenticator like Aveneo posted above - simple, cheap, you only pay one time, and they work.