30 vacant lots is too vague to know. I'm guessing that these are undeveloped lots a builder has? If so, it's going to be rough terrain with debris scattered about. Not something I'd take a prized mower on.
If I'm wrong, and these have been maintained...shoot, you should be able to zip through these in no time. There aren't many things I'd do for $21, but you'd be able to do 3-4/hr. I'd jump on that.