The last major bear market in crude oil was in the late 1980s, when a collapsing WTI price cut the legs out from under US independent drillers. The collapse precipitated a crash in Texas real estate and triggered the infamous savings and loan crisis. The US DOE's EIA maintains a database of historical WTI prices. The absolute bottom in that 1980s bust was when the WTI spot price touched $10.83 on July 23 and July 25, 1986. I had to eyeball the data series but I'm pretty sure I didn't see any lower price. Someone with more time on their hands is welcome to search for a lower price at any other time in US history.
Let's find out what the equivalent of that price would be in today's inflation-adjusted dollars. Using the Minneapolis Fed CPI calculator gives us $23.69 in 2015. Using the BLS CPI calculator gives us $23.39 in 2014, the most recent year available. Kudos to the Fed for getting the CPI adjustment even closer to the present day than the BLS. Anyway, this gives us an idea as to how much farther the WTI price can fall before it touches a truly historic bottom. The WTI Cushing spot price for February 23, 2015 (the EIA's most recent date) was $49.56, more than twice the inflation-adjusted 1986 bottom.
The price of oil does not have to fall that far before it recovers. Alternatively, it could fall even farther and break all historical records if demand destruction continues in the developed world and US shale producers keep pumping rather shut down in an orderly manner. The global supply glut of oil will not close to meet demand at a natural equilibrium until at least some time later in 2015. Investors hunting for bargain prices among oil producing companies are not waiting that long to begin their search.