With no limits on how long it takes? I suspect it may be a cheap LM only ticket to Euston, then walk to Liverpool Street (allow plenty of time - you may want to use Oyster instead

) and then a NXEA only ticket (as described) from there.
If you are returning same day, there is the useful Prestatyn to Nottingham CDR ticket as recommended by Ashworth gets you half way at a cheap price, but it's difficult to see how you'd get much time in Cambridge by the time you got there!
What time do you need to be there by? Are you staying overnight?
If you wanted to travel at peak time then the +Any Permitted fare of £54.50 isn't bad, as that's valid on any train. Cross Country want a fiver more before 09:30 for a, quite frankly, rubbish route on local stoppers. They don't deserve your custom as they clearly don't want it.