How much should I sell the resale rights to my ebook for?

I've been selling an ebook exclusively on Clickbank in a small niche for nearly two years now and have been approached by a top affiliate to buy the resale rights for my ebook. He wants to host a copy of the ebook on his own site and process payments directly (thereby cutting out Clickbank and any commissions).

Some points that could influence my decision:
* Ebook sales have been steadily increasing since launch (i.e. niche still seems profitable)
* It's my main online income source (nothing big but nice extra income)
* Ebook was developed with direct sales in mind (via Clickbank) rather than offering resale rights to others
* I have a royalty agreement with the expert who contributed to the ebook (agreement doesn't mention reselling on other sites)

My questions:
* Should I sell the resale rights to this affiliate?
* Are there other things I need to consider before offering resale rights? E.g. affect on other affiliates, potential customers, setting up a resale rights business model, etc
* How will any royalty agreement with the expert be affected?
* How much should I sell it for? Any formulas such as: Current monthly affiliate sales x Number of months, etc

Sorry for the long post, but any feedback would be much appreciated.

Thanks!