Ready to upgrade to the iPhone 4S? You can take a little edge off the steep prices ($399? With a new contract?!) by trading in your old iPhone for cash. There are a bevy of online sites that'll take older iPhones off your hands—each offering very different prices for Apple's different iPhone models. Luckily, we've done all the price-comparison legwork for you.
We valued several old iPhone models—every phone from the original 4GB iPhone to the white 32GB Verizon iPhone 4—through each of the 5 biggest trade-in dealers: eBay Instant Sale, Best Buy Trade-In, BuyMyTronics.com, Gazelle.com and NextWorth.com.
In each case, we asked the site to estimate the trade-in value for a phone in "excellent" or "like new" condition, with all of the original cords and packaging included.
Your mileage may vary, of course. Expect to earn a little less if your phone is in poor shape or missing any accessories. Also, these trade-in prices are likely to change day-to-day and even hour-to-hour. As demand goes down or supply goes up (like after the iPhone 4S gets into consumers hands), these trade-in values will probably drop. The best time to sell off your old iPhone is right now.

iPhone 2G (Original)
iPhone 2G 4GB
eBay Instant Sale: $66.50
Best Buy Trade-In: $66 (gift card) or $43 (check)
BuyMyTronics.com: $24
Gazelle.com: $22
NextWorth.com: not listed
iPhone 2G 8GB
eBay Instant Sale: $62.50
Best Buy Trade-In: $64 (gift card) or $41 (check)
BuyMyTronics.com: $40
Gazelle.com: $23
NextWorth.com: $50
iPhone 2G 16GB
eBay Instant Sale: $68.75
Best Buy Trade-In: $64 (gift card) or $42 (check)
BuyMyTronics.com: $34
Gazelle.com: $24
NextWorth.com: not listed

iPhone 3G
iPhone 3G 8GB
eBay Instant Sale: $121.25
Best Buy Trade-In: $105 (gift card) or $68 (check)
BuyMyTronics.com: $55
Gazelle.com: $55
NextWorth.com: $71
iPhone 3G 16GB Black
eBay Instant Sale: $141
Best Buy Trade-In: $64 (gift card) or $41 (check)
BuyMyTronics.com: $75
Gazelle.com: $71
NextWorth.com: $79.42
iPhone 3G 16GB White
eBay Instant Sale: $141
Best Buy Trade-In: $64 (gift card) or $41 (check)
BuyMyTronics.com: $75
Gazelle.com: $71
NextWorth.com: $135

iPhone 3GS
iPhone 3GS 8GB
eBay Instant Sale: $179.75
Best Buy Trade-In: $139 (gift card) or $90 (check)
BuyMyTronics.com: $107
Gazelle.com: $116
NextWorth.com: $121.11
iPhone 3GS 16GB Black
eBay Instant Sale: $188.75
Best Buy Trade-In: $80 (gift card) or $52 (check)
BuyMyTronics.com: $127
Gazelle.com: $121
NextWorth.com: $135
iPhone 3GS 16GB White
eBay Instant Sale: $188.75
Best Buy Trade-In: $80 (gift card) or $52 (check)
BuyMyTronics.com: $127
Gazelle.com: $121
NextWorth.com: $135
iPhone 3GS 32GB Black
eBay Instant Sale: $229
Best Buy Trade-In: $156 (gift card) or $102 (check)
BuyMyTronics.com: $138
Gazelle.com: $148
NextWorth.com: $165
iPhone 3GS 32GB White
eBay Instant Sale: $229
Best Buy Trade-In: $156 (gift card) or $102 (check)
BuyMyTronics.com: $138
Gazelle.com: $148
NextWorth.com: $165

iPhone 4
AT&T
iPhone 4 16GB Black AT&T
eBay Instant Sale: $325.50
Best Buy Trade-In: $267 (gift card) or $173 (check)
BuyMyTronics.com: $186
Gazelle.com: $186
NextWorth.com: $200
iPhone 4 16GB White AT&T
eBay Instant Sale: $325.50
Best Buy Trade-In: $267 (gift card) or $173 (check)
BuyMyTronics.com: $186
Gazelle.com: $186
NextWorth.com: $200
iPhone 4 32GB Black AT&T
eBay Instant Sale: $357.50
Best Buy Trade-In: $275 (gift card) or $179 (check)
BuyMyTronics.com: $186
Gazelle.com: $201
NextWorth.com: $215
iPhone 4 32GB White AT&T
eBay Instant Sale: $357.50
Best Buy Trade-In: $275 (gift card) or $179 (check)
BuyMyTronics.com: $186
Gazelle.com: $201
NextWorth.com: $215
Verizon
iPhone 4 16GB Black Verizon
eBay Instant Sale: $297.75
Best Buy Trade-In: $267 (gift card) or $173 (check)
BuyMyTronics.com: $136
Gazelle.com: $154
NextWorth.com: $165
iPhone 4 16GB White Verizon
eBay Instant Sale: $297.75
Best Buy Trade-In: $267 (gift card) or $173 (check)
BuyMyTronics.com: $136
Gazelle.com: $154
NextWorth.com: $165
iPhone 4 32GB Black Verizon
eBay Instant Sale: $318.50
Best Buy Trade-In: $275 (gift card) or $179 (check)
BuyMyTronics.com: $143
Gazelle.com: $160
NextWorth.com: $172
iPhone 4 32GB White Verizon
eBay Instant Sale: $318.50
Best Buy Trade-In: $275 (gift card) or $179 (check)
BuyMyTronics.com: $143
Gazelle.com: $160
NextWorth.com: $172
(Images: Wikimedia Commons, Apple)

Z2 iPod Dock and Wi...
So Best Buy thinks an 8GB 3GS is worth $60 more than a 16GB one? That's gotta be wrong.
It's based on supply and demand. If less people are trying to trade in 8GB 3GS phones, Best Buy's going to be willing to shell out a little more. Meanwhile, folks with 16GB phones are practically throwing them at Best Buy, so their trade-in offer will be less.
Will they take my 3gs with a cracked all over case, sorta-working mute button, replaced screen, wishy washy GPS, and broken proximity sensor? :)
Anyone know of a similar option for Canadians?? None of the above take phones from outside the USA.
Also helpful to jailbreak/unlock your phone before selling it. I've been using iPhones since the first one and have yet to sell one for less than I paid. Essentially I use the phone for a year, sell it for a profit, and use that money to buy a new one. I also keep the original accessories and my phone is in brand new condition.
@Jason, we will take it! You probably won't get the prices quoted above, but we accept electronics in all conditions!
JulieMediaGal
Marketing, Media & Community Outreach Coordinator
Last October, I got an email from AT&T that said if I "surrendered" my original iPhone 2G 4GB and renewed my contract I'd get a free, FREE, iPhone 4. I was at the store in that afternoon. Best deal ever.