Beachie -
If this were my name, I would be around $750 - $800.
$450 shows that they are interested and are serious about getting the name. I would emailthem and say "My target was about $1000, but I would split the difference with you at $750." Make sure you point out the free and immediate transfer. Also, offer to add another year of registration to the name for them . . . Hopefully that will get them over the edge.
Good Luck !
-Bob