Bounty history can be tracked with the general reputation audit. Check out http://stackoverflow.com/reputation (or whichever site is appropriate), and check the day in question. You'll see a big positive number for the bounty, considerably larger than any other number as receiving a bounty is the only method to earn more than 15 points in one shot. Alternatively you can just skim for vote code "9", which is BountyClose.
Reviewing what mmyers points out, if you're looking for the reputation lost from a bounty (which will occur when you originally place the bounty, not when someone else gets rewarded), you'll just look for a similarly large negative number. The vote code for BountyStart is "8", you actually need this number if the bounty cost you 100 because it is possible to incur -100 for a post deleted as spam/offensive. I don't know the vote code for those, but it's definitely not 8!
Simply take the reputation from the day before, add up all the values up until the bounty, and you'll have the reputation before the bounty. Just add the number afterwards to get your reputation after the bounty.