Can a crown be repaired, if so, what system is used? My dentist wants to replace the crown, cost $800.

bmccrack,
It depends on how and where the crown is broken and the age of the crown. If porcelain has broken off or chipped, sometimes it can be repaired with a kit borrowed from the lab if the dentist doesn't have access to one in office.

If the crown is rotten underneath you should get it replaced... if it is leaking you can get it repaired with a filling (drilling a bit out and placing a filling) but its not ideal. Porcelain repair doesn't work that good in the long term especially on back teeth (lots of force) so if the porcelain is broken maybe you can get it smoothed off but it may keep chipping... a full gold crown is the best bet if you want the most durable product