If you're a Magpi Free user, you don't need to worry about the cost of uploads, as long as you're not exceeding the limits of the free account: no more than than 500 records (completed forms) to our servers each month (so no more than 6000 per year).
For paying users (with Pro and Enterprise accounts), their uploads work like prepaid mobile. The subscriptions come with a number of included upload credits (500 for Pro monthly, 10000 for Pro annual, and 20000 for Enterprise). As the paying users upload records, they are reducing their upload credits. Once their upload credits drop to zero, they will be unable to upload more until they "top up" (recharge) their accounts by buying more credits.
And don't worry: we'll send an email to the registered contact emails for your paid account when the number of upload credits remaining drops to 2000, to 1000, to 500, and to zero -- so that your work flow is never interrupted. More information about the general costs of Magpi is available here.