I think the card is $15 plus however much credit you want to put on it. They have a number of calling plans, one of which is 10c/min to landlines and other Cingular mobiles, another of which is free calls to all Cingular mobiles (handy if you've got two -- better than two-way radios in the parks) but $1 a day service charge. You'll have to ask in the shop for the current plans. Calls back to the UK were pretty cheap too -- something like 25-30c a minute, rather than the nearer £2 that Orange want...
To unlock SonyEricsson phones you need a special cable, or someone who has one. But if you have a Nokia just do a Google for DCT3 or DCT4 unlock codes. Don't pay for them -- click on all the links until you find a free one. For other phone manufacturers I'm not sure. Of course your phone may be unlocked already -- try a SIM from a different company (borrow from family/friends/colleagues) to find out.