Fun fact, not buying doesn't increase main chances.

Basically how it works is the more floors down you go the bigger the chance of mains. I did a few experiments while soloing.

When I bought I got a main on 5, 7, 8 and 10.

When I didn't buy I got a main on 5,7,9, and 10 (Not including Dandy on 6).

So don't bother with not buying when trying to get mains. If you want Dandy then sure, don't buy, but it's not necessary for the rest of the mains.