Flesh has a density similar to water - the ratio of fat to muscle will alter this a little, but it will still be close to water.
Most of the rock-like minerals you would think of when thinking "turned to stone" have specific gravities ranging from 2.5 to 3.0 - granite is 2.6 - and since flesh = water (which is the standard 1.0 on the specific gravity scale) this is a simple multiplication of weight.
If you want to be really fussy, the metal things that the character may be holding will actually get _lighter_, since the specific gravity of iron is about 7.8.
So, 180-lb warrior turns into 468 lbs of granite (granite is 2.6), and his 60 lbs of metal armor and weapons turns into 20 lbs of stone armor and weapons (7.8 iron is 3x as heavy as 2.6 granite) for a grand total of 488 lbs.
Figure 300 to 600 lbs then, depending on character size.
I found a chart of specific gravities of various things here:
http://www.reade.com/Particle_Briefings/spec_gra2.html
So if you want to turn somebody into butter (0.87), cork (0.24), garbage (0.48), pitch (1.15), or tungsten (19.62), now you can calculate their weight exactly.
