I believe your counter offer will negate the original $200 offer but…… I believe you may just ask a question of the bidder how he/she justifies the offer and could they go higher. I know there is a comments response and have used it but since I have no current offers on my domains cannot access it.
Look for a comments option.
Additionally I believe $200 to $400 is a reasonable price expectation since most of the other extentions are just parked or coming soon or not even regged.
Cheers!
WineGuy