one place not mentioned that is cheap(er) than City car parks is the Garden House Hotel - behind Peterhouse, next to the University Centre.
Apart from that - yes, it's already been said, but just to clarify -
1) UL Car Park - never checked, left car many-a-day there - only problem is that it's an arsey 15 min walk from town.
2) Herschel Road/Adams Road - both forming a 'circle' round Robinson. Free parking 24/7. Look it up on Google Maps.
3) Evenings/Sundays - the area around Catz is good. Queens' Road has free parking on Sundays, as does Kings Parade, as does the dodgy small roads round the back of New Museums/Corpus. Actually, if you're willing to risk it, New Museums is rarely (if ever) checked and a good 'free' central car park.