That's a hard question to just give an answer.

Depends on so many things.

Like your offer, your conversion rate, Even after
you have done the best keyword research, so
many things have to be in place to figure ROI.

Are you selling your own product? Are you selling
some one elses product where you have no controll
over how the sales letter converts.

I'm sure you will get more insiteful responces.

I will follow the posts, along with you.