Your old servo may be able to fit the new c/ku feedhorn, but you're right they are very expensive. If you do buy a new c/ku feed, you're better off with c and ku band lnbf's that don't need a servo.
Hi Kelly,
What you want is a dual band feedhorn. I use a chapparal corotor that i bought off of flea bay for 80$ that came with 20 degree c band lnb and .7 ku lnb. You can find package packs that have everything including a new servo for about $200 (new).