I've been doing concert photos for various artists for a non-profit organization for a sequence of summer events. The original agreement was that I would be doing all the photos for free, but if they were to be used on any site they published, that I obviously should be referenced. They sent me an email asking to purchase the digital rights to a subset of those photos, which I assume means they do not need to reference me and can do whatever they wish with those. Another thing is that those photos cannot be used or referenced by myself without the permission of that organization.
My question is: what should I do? I am currently an amateur photog moving towards doing it professionally, so I am in the process of building a portfolio. How much do you guys charge per photo if someone were to buy the copyrights to your stuff?