IFor 30$ i can get 300 likes (more or less) It's an average, but depends on your niche, targeting, etc.
I do this by facebook ads, so pople will like because they really like your page.
If you want cheaper likes go ahead, with bots or automated like sellers. It's a waste of money. You'll get a lot of page likes, but no one will engage, no comment or shares your posts likes.