Once you've gathered a basic amount of ratings, reviews and feedback and that the general consensus among readers is that your guide is so amazing that they'd pay for it, why not start charging for it?

It's best to keep the price below the cost of the average book to begin with. You want to make sure that your first real customers find enough value for their money and that the guide meets their expectations and needs (this also starts with a good marketing description, so spend enough time there too).

The ideal range (again, this is just a guideline because it does vary from guide to Guide due to subject, content depth and writer experience) is $5-$20. If new readers are willing to pay this amount and continue to convert at the same rate you know that you're onto something.