http://cnet.phonedog.com/cell-phone-research/compare-plans.aspx

hope it helps

You generally won't find discount codes for rate plans, although some corporations have discounts with specific carrier(s). Usually these come in the form of a percentage (e.g., 15%) off your monthly bill.

1. Price per minute – prepaid cell phones minutes cost between 10 cents and 60 cents. That's a huge range. With some prepaid providers you buy a bucket of minutes, and the more you buy at one time, the less per minute cost.

With others you put dollar amounts on your phone. With more money you add, your number of minutes per dollar will increase. Long story short, the more you spend, the cheaper each individual minute will be.

2. Expiring minutes -This is really important. Once your minutes expire, you will lose them. If you use your cell phone often, this probably won't be an issue.

But, if you are buying a phone for emergency purposes or to use infrequently, you might want to spend more per minute for a later expiration date. Some carriers also make you add a certain amount of minutes or money per month in order to keep your account active.

3. Daily Fees -Some carriers charge a fee for each day your phone is used. This can be as high as $1.00 a day, but will come with lower per minute fees.

If you use your phone infrequently this might be a good deal for you. If you use your phone for many days a month, you will be paying too much, especially if it's for short call.

4. Nights and weekends/ mobile to mobile -Some plans offer free nights and weekends and some offer free calls to others with the same carrier.

Again, you might want to pay more per minute if you will be able to take advantage of these features.

5. Roaming charges – Some carriers have roaming charges and other have none. It is important to find out if there will be times that you will be roaming.

If you will be roaming frequently, you will want to avoid a carrier that has roaming fees. Some are up to 70 cents per minute.

6. International calling -Some prepaid plans offer it and some do not. If you need international calling capabilities you will want to be sure that the carrier you select offers this service. Finding the best plan can be a daunting task.

Be sure to sit down and write what your needs will be before shopping. If you know what your needs are for the above topics, you will be able to make an educated comparison of prepaid plans.

Let's say plan A is 39.99/month for 500 minutes, and then it is $0.65 per minute after you use the 500 minutes.
Let's say plan B is $34.99/month for 450 minutes, and then it is $0.60 per minute after the 450 minutes.

Let x = number of minutes you use each month.
The cost of plan A would be
C = 39.99 if you use less than 500 minutes or
C = 39.99 + 0.65(x-500) if you use 500 minutes or more.

The cost of plan B would be
C = 34.99 if you use less than 450 minutes or
C = 34.99 + 0.60(x-450) if you use more than 450 minutes.

Right away you can see that Plan B is cheaper if you don't use more than 450 minutes.
You can find when the plans cost the same amount by setting the cost of each plan equal to each other.
39.99 + 0.65(x-500) = 34.99 +0.60(x-450)
Then solve for x.