My theory is to pay as little as possible (as a dad, I suspect your dad may appreciate this). The question then is which silly features are important enough to you to merit the cost.
Do you want one of those full tiny keyboard things?
Do you want a touch screen?
Do you want a camera?
Are you going to be want to use the internet on this thing... and if so, for what?
Will you use bluetooth (i.e. for those geeky hands-free headset things).
To get the answer... you should really write down a list of the features you want. I would suggest dividing the list into "must have", "important" and "cool but not necessary" sections. If you do this, then picking the phone will be a lot easier.
(for the record, I carry a Virgin mobile phone. To buy the phone outright (i.e. no contact or commitment) cost me just under $20. I pay under $20 bucks a month for the few minutes I use .)