As Chris points out, the initial purchase price is rather irrelevant in a contract context, with the total cost of ownership of the phone (in extreme cases) sometimes exceeding $3000 by the end of your 24 month contract. So $100 might as well be $0 up front.
But the other big take Americans could get from us Europeans is that there's another way, one that is almost unheard of in the carrier-dominated USA. Rather than buy a phone on contract for 'free+£35/month for 24 months', which works out at £840 all in, you can buy the same phone for £350-SIM free and then add on a pay-as-you-go SIM and data rolling-monthly-allowance (from £10/month for unlimited data if you shop around), meaning that even with quite a bit of Internet use, you might save 30% or more on the contract-inclusive price.
The situation's certainly not cut and dried - if you're a heavy talker or texter than a contract still makes sense. But I fail to see why more people don't talk about the attractions of not having a contract at all.
And gaining the advantages of no carrier branding/crippling, faster access to firmware updates, freedom to sell whenever you like, and so on.
Just another viewpoint! Maybe an American reader can give us a pay-as-you-go perspective - am I missing something?