It depends. You have two choices. You can either purchase problem credits or unlimited credits for a set number of students. In some cases it’s cheaper to pay a fraction of a cent per submission. In some cases it’s cheaper to get unlimited submissions for a set number of students.
If you go with credits, you’ll pay a penny or less per submission depending on how many you order at a time. And to go ahead and answer the most common question about credits, no, they don’t expire and they do carry over from month to month. You also get 1,000 credits each month for free, although those do not carry over if you don’t use them.
If you go with one of the unlimited plans then the submissions are still counted, but you’re not billed for each one. Your students can submit as many as they want and it won’t change what you’re charged.