265/35R18 have 6/32nd tread remaining
275/35R18 have 4/32nd tread remaining
Thread depth starts at just 8/32 (they can be run down to basically no tread for autocross) so these are still very usable and very fast. If unfamiliar with these tires they were released Spring 2015 and are just about the best autocross/track day tire out there...even good in the rain. Also totally streetable as many in the club know.
Buying new would be nearly $1500USD shipped just for the tires from TireRack.
http://www.tirerack.com/tires/tires.jsp?tireMake=Bridgestone&tireModel=Potenza+RE-71RJust
$700