How much are the ongoing costs though?

I live in Europe, so it's probably not comparable. Around here you can get a prepaid plan with no mandatory payments, the only requirement being making a paid call every once in a while to keep it active.

T-Mobile seems to have a prepaid plan for $3/month, it's still expensive but not that bad. Alternative, you can just get the phone, then switch the SIM cards if your main phone is broken (doesn't work if you lose it instead, though).