If it were me i'd ask them to make a sensible offer based on the fact you're not in. any urgency to sell and you planned on developing it ( i think you said this  in the other thread ) You could even point them to the domain you mistyped in the other thread as a rough guide on price expectations (That site is a well known professional  domainer)  See what they come back with and whether you'd be prepared to let it go.  IMHO the plural would genarally be worth less than the singular.