A $5 scope is a toy, not a real instrument. They're not useless...but they're toys, just keep that in mind.
Anyway, you should ask yourself: what would you do with the info a scope might give you?
If it gives you a clear view at 400x, you'll be able to count cells, but you'll need a hemocytometer. And you should do viability as well, so you'll need some methylene blue. As for contamination, I'm highly skeptical that a $5 scope will allow you to do immersion-oil viewing at 1000x (gotta use oil immersion at this mag, and yes immersion oil is the same as mineral oil). But even if you get reasonable viewing at 1000x, will you be able to identify what you're seeing?
Use the freshest yeast possible, perhaps make a starter, and clean/sanitize well. Save $5.