iPhone

All
http://www.everymac.com/systems/apple/iphone/index-iphone-specs.html

http://support.apple.com/kb/ht3939

$180
http://www.shopxscargo.com/product_catalogue/cat_product_details.asp?category_id=43&product_code=35493&sub_category_1_id=184&category=Cell%20Phones,
http://www.everymac.com/ultimate-mac-lookup/?search_keywords=A1241

Apple iPhone 3G 8, 16 GB Specs

http://www.everymac.com/systems/apple/iphone/specs/apple-iphone-3g-specs.html, 

http://www.everymac.com/systems/apple/iphone/iphone-faq/differences-between-the-original-iphone-and-iphone-3g.html, 

http://www.everymac.com/systems/apple/iphone/iphone-faq/differences-between-iphone-3g-iphone-3gs.html

3GS

http://www.everymac.com/systems/apple/iphone/specs/apple-iphone-3g-s-specs.html  

$200

http://www.shopxscargo.com/product_catalogue/cat_product_details.asp?category_id=43&product_code=36281&sub_category_1_id=184&category=Cell%20Phones

Apple iPhone 4 (GSM) 

http://www.everymac.com/systems/apple/iphone/specs/apple-iphone-4-specs.html

$300

http://www.shopxscargo.com/product_catalogue/cat_product_details.asp?category_id=43&product_code=37337&sub_category_1_id=184&category=Cell%20Phones

http://www.newegg.ca/Product/Product.aspx?Item=N82E16875100023

http://www.sellyourmac.com/mac-product-guides/iphone/sell-your-used-apple-iphone-4-gsm-8-16-32-gb-mc318lla.html