Wow, it just so happens I'm looking at changing from an ATT contract-style to a pre-paid plan (probably from Straight Talk) right now! You're looking for numbers? I can give you some numbers (it's easy enough for you to get this same information, too, as it applies to your exact needs).
I currently have ATT, but have an old phone and am not under contract. I'm looking to get two smart phones, and upgrading to unlimited talk, text, and data.
From ATT:
Their best choice (for my needs) is the Mobile share plan. Unlimited talk, text, and data (ATT doesn't offer "unlimited" data so, I'll use 4GB, shared, as a comparison) is $150 / month ($70 for data, + $40 per phone).
I can get a Samsung Galaxy S4 from ATT for $200 (times 2) with a 2-year contract.
That works out to a total of $166.67 / month over two years.
From Straight Talk:
Not necessarily the cheapest, but seems like the best value for my needs and I can be on the exact same ATT network. Unlimited talk, text and data. They claim it's truly unlimited, but I see conflicting reports of possible "soft" limits placed on very heavy users. I picked 4GB from ATT as a comparison.
Service for $90 / month ($45 per phone, two separate plans).
You can buy a new Samsung S4 for around $600, sometimes less. Let's spread that out (times two) over the same two years.
That's a total of $140 / month.
The difference? At first, you save $26.67 per month with pre-paid, not a huge difference, but worth considering. But wait a minute, once the contract is over, then the difference is considerable at $60 per month. You're still paying ATT for the phone subsidy!
Also, there's more. From ATT, there are a ton of telecommunications and sales taxes, totaling possibly around 20 to 25%. For prepaid, you generally pay your local sales tax and nothing more. Let's say that adds another $25 to your savings with prepaid. That's maybe $85 per month savings.
With Straight Talk, if you pre-pay in larger chunks, you can get it down to $42 per phone. Sometimes you can get refills on ebay, etc. even cheaper.
Of course, if you're happy with a less expensive phone, you can save even more with the prepaid.
If you're the type that always upgrades to the latest and greatest phone, then the difference isn't quite as large, but if you already have a phone, don't need to upgrade frequently, or can buy a less expensive phone, then prepaid becomes really attractive.
Also note, there are differences in the details. ATT includes free tethering. Straight Talk forbids tethering altogether. Straight talk supposedly offers truly unlimited, but has more restrictions against streaming, etc. On the other hand, if you go above 4GB with ATT, the cost gets progressively higher to the point of absurdity.
This is just one example. You can do the math yourself with any big contract provider and pre-paid service. Generally, you'll get the same results.
Boost mobile has even more affordable pre-paid unlimited, but for me, their Sprint network has poor coverage. Your mileage may vary. If you're a light user, Consumer Cellular has very inexpensive plans, but they don't offer unlimited service. Republic Wireless offers a $19 / month unlimited plan, but routes your calls over wi-fi (not always, but whenever possible). It has performance quirks, but for the very budget-conscious may be worth considering.
If you are a very light user, especially if you don't need a smart phone, you can buy blocks of minutes and get your rates down to less than $10 per month, possibly a lot less.
I suggest you Google "best prepaid cell phone plan" or "$10 cell phone plan" or "cheapest cell phone plan" something similar. There are a lot of providers out there.
Do be careful. If you buy your own unlocked phone, there may be many versions of the same phone, even for the same carrier. Make sure you're comparing apples to apples.
Last, let me make a disclaimer. I don't work for any of these companies and have no interest in them at all. I do have ATT but haven't actually tried any of the other services that I mentioned.