You're in luck; John Hinderaker of Powerline did the work, looked up Santorum's rating by the American Conservative Union and found that Santorum ranked just about in the middle of the 2006 Senate, the last year he served. See what Hinderaker writes:
Santorum was not one of the most conservative senators. On the contrary: while 20 Republicans had voting records that the ACU rated as more liberal than Santorum’s, 26 had voting records that were more conservative. (snip) While Santorum was a reliably conservative senator on social issues, he was not very conservative, for a Republican, on economic and fiscal issues.