Looking at the companies involved with the term "Plan A" - I'd take no less than $1,000 for it, they can afford it. And if they stop following up, their interest in the name won't change a week from now if you email them back and say you reconsidered and will accept their offer, it's still their brand. (No less than a week though)
If your offer was $xx - low $xxx, they're either an enduser lowballing you or a domainer wanting to sell to one of those endusers for $xxxx
Also, google their email address and see what comes up, see if you can link them to any one particular site to get a better idea of the type of budget they're working with.