@jlyg just to add to what @will13am noted: there is no pay-per-use billing available at all. You get what you pay for, and you pay for what you get [up front], period.
EDIT: if you're a very occasional mobile data user, using the add-ons like @will13am noted provide okay value even though they are a bit expensive to purchase up-front. They are not tied to your plan or cycle and remain on your account until they are used up, so you don't have to worry about them expiring. If you are a careful and occasional user, a 1GB add-on could last you quite some time.
On the other hand, if you anticipate needing data fairly regularly, it might be cheaper in the long run to look at a plan that includes data already. For example for $30 per 30 days, you could get unlimited calling and texting instead of only 50 mins and 50 outbound texts, plus 500MB (of slighlty slower, "3G" speed data that's fine for most use). Yes it's $20 more per "month" than the $10/50/50 plan, but if you're buying add-ons more often or running into limits on the calls or texts, it may provide better value.
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