If you are starting a new 2-year contract, the price for the phone itself (8GB) is $199.99 plus tax.
There are regular monthly fees for your phone service that come with the iPhone. At this point in time, they are not selling iPhones WITHOUT voice/data plans.
Basically, for an individual, plans start at $69.99, and that includes 450 anytime minutes, unlimited mobile to mobile, and 5000 nights and weekends. In addition to that, it includes UNLIMITED data and visual voicemail (meaning internet usage, basically). You MUST get both voice (minutes) and data (internet) to have an iPhone at this pricepoint. The $69.99 covers all of the services mentioned.
That price does not include text messaging. Depending on how many texts you want, the price will increase accordingly.
Basically it is like any other standard phone, where you pay for your plan, any extras (such as texting), and taxes and fees. These are your monthly payments.
But again, the phone itself is $199.99 for 8GB (black only), or $299.99 for 16GB (black or white). And again, the plan price includes the cost of internet use and visual voicemail (no getting out of that), just not text messaging.
If you are wondering why you MUST get a data package with the new iPhone 3G's (as opposed to the older ones, where data was optional and extra), it is because they are concerned about overage. Data is built into the cost of the plan to avoid people racking up massive phone bills from going on the internet without a data plan.