The 1335 National Express coach runs directly between Coventry & Cambridge, arriving at 1650.
The 1550 service is also direct, but faster, arriving at 1830.
The single fare, on either service, is �17.70 (+50p booking fee). Other services are available, changing at Milton Keynes.
http://www.nationalexpress.com
There are no convenient rail services linking the two cities. You could travel by rail to Milton Keynes Central and take the X5 bus to Cambridge, but it would be more expensive than travelling by National Express.
Chris