Grinding is doing something over and over again until you feel satisfied by some arbitrary basis. When students who've made it a ways through decide to go back to the beginning, they still follow the assigned numbers of pages, so it doesn't fall under that category. The 250 box challenge itself is a repetitive exercise, but it has a clear finish line.

As to how much each lesson costs, you can see the prices by going to the submit page and selecting the different lessons with the "Submit for Official Critique" option toggled. That said, it's a pretty straightforward breakdown - Lessons 1 and as well as the box challenge, cylinder challenge and wheel challenges are all 1 credit each. Everything else (including the texture and treasure chest challenges, both of which are optional), are 2 credits each.