Turns out I had unneeded ) after each instance of .88, but needed one additional one at the very end. A co-worker spotted the error.
=IF(ISNUMBER([@[12-Month Lock-in Amount]]), [@[12-Month Lock-in Amount]]*0.88, IF(ISNUMBER([@[6-Month Estimate]]),[@[6-Month Estimate]]*0.88, IF(ISNUMBER([@[30-Day Estimate]]),[@[30-Day Estimate]]*0.88, "")))