If your budget doesn't allow you to get a sparkling new iPhone, why not go for a used one? You can actually save a lot of money and get a nice used iPhone, just as good as a new one.
Even the older model, the 2G iPhone, has it's advantages like e.g. a better autonomy.
One of my friends actually went for a used iPhone, but she wanted it unlocked, and she couldn't find any at the ordinary phone stores. I told her to look on eBay, which is in fact the obvious place to look for a used unlocked iPhone. There would normally be a lot of different onces to choose from. Some has extra accesories, some hasn't.
I would go for one where you can see an image of the used iPhone, and I would look at the seller too, to see his reputation. Close to 100%? Then I feel absolutely safe.
So if you are looking for a used unlocked iPhone, eBay would probably be the right place for you too, to look. Below here you might find what you are searching for:
 Apple iPhone 3G 8 GB Black Unlockable  US $320.00
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 Apple iPhone 3G 8 GB Black UNLOCKED ATT T Mobile US $215.50
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 Apple iPhone 3G 8 GB Black ATT US $152.50
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 Iphone 3GS 32GB WHITE UNLOCKED US $522.00
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 APPLE IPHONE 3G BLACK 8GB UNLOCKED MINT FREE SHIPPING US $294.00
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 Samsung T929 Memoir Unlocked Many Acc Free Shipping US $227.50
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 Apple iPhone 3G 16 GB Black ATT MINT unlocked 3gs 2g US $310.00
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 Brand New In Box Apple iPhone 32GB Black 3GS Unlocked US $730.78
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 Brand New In Box Apple iPhone 16GB White 3GS Unlocked US $643.09
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 Apple iPhone 3GS 16GB White 3 months old Unlocked US $255.00
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 IPHONE 3G 8GB BLACK 312 UNLOCKED JAILBROKEN INTSHIP US $305.00
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 APPLE iPHONE 3GS 32GB NEW IN PLASTIC BLACK UNLOCKED US $730.08
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