I think you can be charging much more than you are, but if you're working with smaller businesses then maybe that budget is more reasonable.

Why not offer some kind of tiered option? A post is $15, highlights are $10, but if you get both they're $20—something like that. If you other avenues to distribute your news (like a newsletter, for example), you can add additional packages.

I also think 3:1 is too many ads but if your audience is still engaging and there isn't a noticeable dropoff, that's not too bad.