They haven't published any exact information as to difficulty level, unfortunately. I just quickly scanned through and I'd say the last 20-30 are in the 700+ range. The maybe 30 before those are in the 650-720 range. 600 range looks like it starts in the high-double-digit numbers (maybe 70 or 80).

These are, of course, just estimates - you can't really tell just by looking what the difficulty levels are. We run the data on our own questions (based on our students' results) and we're sometimes surprised by the difficulty level that comes back for a particular question.

Do remember one thing: you don't want to study just those problems that you can't do easily yet, or just those problems that are at the level that you want to score. You still have things to learn from problems that are right in your current scoring range. How can you do the same problems more efficiently? How can you build on those skills to get to the next level of problems of that same type? Etc.