How much do I need to put in ankibrain

I wanted to use ankinbrain to generate cards for my med school, because they kinda take too much time to write each one But in ankibrain you need to put some money in it so u can generate cards, it says 1$ for 3000 pages, but I read a post that said it took 0.5$ for a document of 30 pages So how does it work, and how much money should I put in it?