What to ask???

Hi guyz,
I've a blogger blog about books and a couple of people just asked me to review their books.
My first thought is to ask them some $ (let's say 50 $ per book review) but I'm worried because my blog has not a great traffic (in fact they contacted me via twitter). I'm talking about 20 people per day..., even though my blog is on the first page of google for an important keyword related to books...
Anyway, I don't want that they could do bad advertising if the get offended by my money request.
What do you suggest?
Refuse (you know a book review is really long an hard) or ask $$$?

You have many options, one will be to tell them that you will do it but there is a line and they can pay to get ahead in line.

You can offer packages of review and advertising on the site, twitter...

Hi kaplan, very nice for the replies.
But what if they ask me about my traffic?
And which should be "the line"? And what do you mean for packages of reviews???