This would in and of itself be expensive and hazardous, upping the asking price significantly. Keep in mind that this hypothetical dragon egg merchant, leaving aside his dubious moral outlook which permits him to traffic in intelligent creatures, has to be a) knowledgable in dragonkind and b) have the means and patience to keep the egg vital. If someone in one of my campaigns put me on the spot, I would say 50k gp, but perhaps varying by size and/or other factors, so maybe 40k for a White, 60K for a red, for instance.
IIRC, it does not provide rules for cost of eggs, but I would expect them to be no less than tens of thousands of gp. The Draconomicon has excellent information pertaining to gestating, hatching and raising dragons as companions, familiars, etc.
