If we assume that Iron Man is a spherical mass of iron
a man-sized object completely composed of iron, we can set an upper limit to his mass. I estimate that his volume couldn't be more than about 150L (or about 39 U.S. gallons). (As an aside, a person of this volume would weigh about 150kg, or over 330 lbs.) A 150L object made from iron would weigh about 1180kg, or 2600lbs. Of course, there is a squishy Robert Downey Jr. inside that iron, with a volume of approximately 70L, so we need to reduce the total accordingly. (1180 - 70*(7.87-1))= roughly 700kg, or a little over 1500 lbs.
The armour isn't made from iron, though. Here's a line from the movie, speaking to JARVIS:
Tony Stark wrote:Have it reconfigure the shell metals. Use the gold titanium alloy from the seraphim tactical satellite. That should ensure a fuselage integrity while while maintaining power-to-weight ratio.
Again, if we assume that gold and titanium the only constituents of this alloy, and further that they are in approximately equal proportions, then we have an alloy which has a density which is actually higher than iron: (19.32+4.506)/2=11.913, or roughly 50% more dense than iron. So for an upper limit, we're at 1500*1.5=2250 lbs (or 1050kg). (I'm not a materials guy, so I'm sure that this is a naive, simplistic analysis. Best I can do on short notice.)
In any case, there's no way that Iron Man can weigh "tons". Unless by "tons" we mean "more than one ton", because that extra 50 pounds isn't much to hang your hat on.
Edit: Sorry, Hav, I had to calculate before I could bask.
73% of all statistics are made up, including this one.
I'm waiting as fast as I can.