Let's start with the basics:

mescaline (freebase) = C11H17NO3.  

C  11 * 12.011
H  17 *  1.008
N   1 * 14.007
O   3 * 15.999    
       -------
total  211.261 grams per mol mesc

This would be mesc as the freebase; an oil at STP.

As the stable, solid forms, there are two common ones: the
hydrochloride (HCl) salt and the sulfate (H2SO4) salt.  It possible
that someone has made, say, mesc acetate, but it's not something I've
ever seen.  They're on their own.

With the hydrochloride, a single mesc molecule is paired with one HCl
molecule to form the salt.  

so, some more masses:

Mescaline Hydrochloride:

mesc  1 * 211.261 
H     1 *   1.008 
Cl    1 *  35.453 
          -------
          247.722 grams per mole mesc.HCl

Sulfate: In the sulfate, there are two mescalines per H2SO4.  To 
complicate things slightly, the sulfate exists as the dihydrate, 
meaning there are also two H20s per unit cell.  You end up with:

     (mesc)2.H2SO4.(H2O)2

mesc  2 * 211.261
H     6 *   1.008
S     1 *  32.068
O     6 *  15.999
          -------
          556.632 grams per mole of (mesc)2.H2SO4.(H2O)2

But you want that in terms of one, not two, mesc molecules.  So divide
by two: 

          278.316 grams per mole mesc sulfate [mesc.(H2SO4)1/2.H2O]

		
Conversion factors based on the above, for equivalent amounts of
mescaline:

100 mg mesc sulfate = 89.0 mg mesc hydrochloride = 75.9 mg mesc freebase

So, with the two salt forms, there's only about an 11% difference.  If
someone gets confused between one and the other, the mix-up is not
going to cause major havoc.