Bankrate.com has a FICO calculator which was created by Fair Isaac.
Credit Score Calculator | Improve FICO Score
I tried putting in the following: 1 credit card, opened 5 yrs ago, no mortgage, no applications in last 6 months, balance of $5K, representing 50-70% of limit, no delinquencies. Score range was 650-700. Then I changed the balance to $0, representing 0% of limit, and keeping everything else the same, and score range went to 730-780.
That seems like proof to me that paying off a $5K balance that represents over 50% of the limit will raise your credit score 80 points or so.