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 Black 16GB Perfect No Reserve Unlocked  US $255.00
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 Apple iphone 4 Black 32GB Factory Unlocked US $960.01
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 Apple iphone 3GS White 16GB UNLOCKED SLIGHTLY USED US $335.00
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 Apple Iphone 3GS 32GB Jailbroken Unlocked 313 Black US $385.00
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 Apple Iphone 3G 16GB Orange Jailbroken Unlocked 313 US $203.58
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 Apple iPhone 3GS 16gb Black 40 NOT UNLOCKED US $270.00
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 iPhone 3G Black 16GB JBKN Unlocked Cases Cables More US $202.50
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 Apple iphone 3G White 16GB ATT or TMobile UNLOCKED US $212.50
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 16GB Apple iphone 3GS WhiteUnlocked MINT PERFECT LK US $405.00
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 New Apple iPhone 32GB 32 GB 4 Black Factory Unlocked US $932.04
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 Apple iPhone 3GS White 32GB iOS4 Jailbroken Unlocked US $405.00
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 IPHONE 3GS 16GB WHITE UNLOCKED IN ORIGINAL BOX US $355.00
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