Really, you'd have to compare the price of buying an iPhone in the Phillippines and buying a phone (or having your friend buy one for you) and paying the contract cancellation fees.
Since AT&T technically won't let you buy an iPhone 3GS without a contract, the only solution would be to have your friend buy it, and then cancel the contract and pay the $175 cancellation fee, and though I'm not sure what would be involved with the monthly bill, you may have some additional charges related to the monthly bill. This would certainly be cheaper than buying one from the Apple store because you'd have to pay the full price.
Now depending on which phone you get, the 16gb or the 32gb version, it will modify the price. Through AT&T, the 16gb is $199 and the 32gb is $299. Add $175, plus allow for any possible monthly service charges, and you'd have your total. So at least $374 for the 16gb and $474 for the 32gb. Compare that to the full Apple price of $599 and $699, that's quite a savings.
Now you'd have to figure out what you'd pay in the Phillippines to know how much it costs there, and if you'd want to cancel your account there (if you plan to use another carrier) and see what your total cost there. If you'd only spend $50 dollars or so more in the Phillippines, it may be worth it just to buy it there, especially since it's going to cost you for your friend to ship it to you, plus insurance, etc.
So there you have it. Find one what it costs to you to buy locally, and go from there.
I wish you luck!