So the formula on this is total military pay, multiplied times 3.1% Then, for every year you don't pay or are working on paying, it accrues interest at a very modest rate. From my historical records:
2008 4.8%
2009 4.5%
2010 3.6%
2011 3.0%
2012 2.6%
Sorry but I don't have anything beyond 2012, but it gives you an idea of the interest accrual over time.
In my case, 4 years of service as an officer resulted in $3800 owed, and after letting a few years pass before making payments, ended up paying a total of $4800 to buy the 4 years. Your $33K seems way out of whack.