That's essentially accurate for US prepaid no-contract phones except for one thing; the credit doesn't last 2 years. With a smaller minute-card purchase like that, the minutes will expire in no more than 90 days, depending on the vendor. (US prepaid providers are more heavily marketing their monthly-fee unlimited-use accounts rather than the more traditional design where you deduct minutes from your total credit. This is because prepaids were very slow to catch on here, and most Americans are used to paying by the month and not having to keep track of how much time they have used.)
Generally, buying the phones online direct from the service provider is the cheapest option; stores don't always carry the very cheapest phones that a provider might offer.
If you wanted to go the SIM route there is only one US carrier that can do it: T-Mobile. Here is the website for their pre-paid US SIM cards:
http://www.t-mobile.com/shop/Packages/ValuePackages.aspx
Each one will cost you about $50 for a month's service, but there is unlimited data included in that.
Be careful re: texting; not all of the very cheapest pay-by-the-minute accounts offer it, and it is invaluable in the parks because of all the background noise.