I can feel a crack at the back of my porcelain caps - can this be repaired and how much is it likely to cost?

Mending or trying to repair
all-ceramic caps/crowns/bridges is not an easy task. The line you feel
may be a crack that is just beginning and eventually could extend all the way
through the porcelain and can compromise the teeth that are anchoring the
crowns. In its early stage, a dentist can place a special adhesive agent
and some resin, almost like epoxy, that will fill this line or crack and slow
down the progression. I would advise seeing your dentist right away
before the line can be felt or seen all the way through the porcelain.
The dentist may also want to adjust the way your teeth hit when you bite to
reduce unfavorable forces on this crown. Cost will be determined by exactly
what is involved in fixing it. 

Unfortunately porcelain fractures cannot be mended and it is likely to get worse over time.  I would suggest you have your dentist evaluate the fracture and see exactly what is going on.  You can then decide if any treatment is needed.  Good luck!