Not having had to stay anywhere in cambridge except my place, I don't have many suggestions, but...
- My gf's boss who lives in London but works in Cambridge tends to stay at sleeperz (www.sleeperz.com) two nights a week - this is at the train station and cheap for a hotel (£55 for a double)
- The company aforementioned gf works for is activehotels.com - they list some guest house/bed & breakfast type places from about £33
- hostelling, eg the YHA
http://www.yha.org.uk/hostel/hostelpages/86.html - but the £18 is per person. You might be able to book a 2-bedded room though (as opposed to sharing an 8).
Hope this helps...
Hugo