correct. ps vita wifi only model is $250 as the 3G/Wifi model is $300.
to use the 3G yes you have to pay for it. the complete pricing has yet to be fully announced. you can get the 3G/Wifi model and not pay for the 3G if you so want to.
this photo shows it is not a locked sim card it is something you will have to get. for the usa though it will only take AT@T sim cards at least at launch.
for usa yes you will need to go threw AT@T to use the 3G. it has to do with that AT@T uses GSM tec. everywhere else uses GSM tec so that the ps vita wifi only model and ps vita 3G/Wifi Model are both region free. also so that they do not have to make many kinds of 3G/wifi models. at least at launch anyway.
it will be a pay as you go plan according to what we know. so you will or should not need to sign a contract for you to use the 3G part.
I will clarify that the design of the ps vita 3G/Wifi model and the ps vita wifi only model are still subject to change. at least until they launch.
the 3G has wifi as well. as the other model is just wifi.