Do not know the cost of a replacement fuser. You could contact metrofuser.com or partsnow.com as both sell replacement on advanced exchange. Another option if handy would be to replace the teflon sleeve. Below is a link to a kit that comes with instructions for repairing your fuser.
http://www.fixyourownprinter.com/kits/hp/K100
http://www.fixyourownprinter.com/kits/hp/K100