It really depends.

First you need to look how good your diet plans or ebooks are. If you are fitness guy or a personal trainer...you can ask quite a lot. I have seen professional diet plans and ebooks up to $120 per report. And that's just a short report what covers dieting and they can customize their own diet plans.

If your ebook is put together from research and free articles...you can't ask much. There are so many recipes out there, so your ones needs to stay out from crowd. You need to make them special.