In my mind the singular would be the slightly better one .. although in this specific case I think the two are much closer than the typical singular/plural domain set. That being said .. I find both to be a little awkward as there's something about the English and/or word-flow that doesn't seem instinctive to me. Personally I definitely would have accepted low $xxxx for the plural (although nothing wrong with doing one counter to see what happens .. but that really depends on how you got the offer and who it came from). Note, I'd also be happy with low $xxxx for the singular, as the actual number would only be slightly higher.
Best of luck with it .. hope you get a nice sale!