Logically, they should offer the same fare as Cambridge Station would
sell for a journey heading the other way, i.e. Cambridge to Heathrow
Underground (NLC0782) if such a fare exists (which you should be able to
check in an East National Fares Manual, or at Cambridge Station).
Failing that, it'll possibly be the cost of an LU Zone 1 to 6 Single or
Return plus the appropriate Single or Return from London to Cambridge.

Railcard discounts *should* be available, as they've been available on
through fares *from* National Rail stations to LU Zones for some time.

Some have indicated in other threads that rail card discounts are available
from tube stations. However the experience does seem to vary. Also the
information on the the railcard websites do not support railcards acceptance
at LUL ticket offices:

Taken from http://www.youngpersons-railcard.co.uk/faq.htm#15

"Please note, however, that you cannot use your Railcard to obtain a discount
when purchasing tickets from a London Underground booking office."

Taken from http://www.senior-railcard.co.uk/faq.htm#12

"Please note, however, that you cannot use your Senior Railcard to obtain a
discount when purchasing tickets from a London Underground booking office."

Couldn't find anything definite either way on the Network Card site.