Never take the first offer, unless it includes provisions for adjustments to actual cost and you're planning to have the work done to rebuild. The first offer will always be low, as with any negotiation.
Secondly, if not planning to rebuild, it probably makes sense to hold out on selling the lot for a while. A lot in the middle of other vacant lots in a fire damaged area will be depreciated compared to a lot between rebuilt homes in a recovered or recovering neighborhood. Also note, there may be some required costs to demolish and remove what is left of the house, which may be required both by the community and also in order to maximize the appeal of the lot.