Suitable antifreeze solution
You are on the right track when you mention molality.  

Because freezing point depression is a colligative property of the solvent, in this case water, all you need to do is to find out which solute has more moles per kg, aka molality.  

The equation for freezing point depression for a non-ionic solute is given as:  

$$\Delta T_\mathrm{F} = K_\mathrm{F}\times b$$

Where:
$\Delta T_\mathrm{F} =$ freezing point depression
$K_\mathrm{F} =$ cryoscopic constant (dependent on solvent, not solute)
$b =$ molality, the moles of solute per kg of solvent

The key point here is that freezing point depression is linearly proportional to solute concentration (molality).  

Since glycerol has a molecular mass of $\pu{91.1 g/mol}$, 1 kg will contain 11.0 moles of solute. Methanol has a molecular mass of $\pu{32.0 g/mol}$, so 1 kg will contain 31.2 moles of solute.  

So, for any given price per kg, methanol will give you $31.2/11.0 = 2.9$ times as many moles of solute, resulting in a 2.9 times greater freezing point depression.  Note that you cannot calculate the actual freezing point depression without knowing $K_\mathrm{F}$.