I would think that in a legalization issue, hard drug usage would increase at a smaller rate than marijuana usage.
Someone who has never tried LSD, cocaine, PCP, etc isn't going to run out to the local drug mart and grab a hit just because it is legal.
The National Academy of Sciences, Institute of Medicine conducted a study in 1999 where they determined that ""In sum, there is little evidence that decriminalization of marijuana use necessarily leads to a substantial increase in marijuana use."
Marijuana and Medicine: Assessing the Science Base. National Academy Press: Washington, D.C., 102.
The statistics from a country where they did do partial legalization follows:
In 2004, 54.1% of the Federal prison population was drug related. I couldn't find state information, I'm sure the percentages are similar. Since 80's, when the War on Drugs went into effect, the percentage has increased drastically year over year:
In 2001 dollars, that 54.1% (on the Federal level only) cost us $1,762,285,944 to house. This doesn't included enforcement or judicial costs. I guarantee that legalization would raise more than that in taxes.