Yes if you had 30 Ml of 36 mg and you wanted 18 mg you would cut with 50% PG or VG, thus giving you 60 ML total at 18mg.
The savings would be in the lower price of your cutting agent.
If your juice is say $20 per 30 ml and PG was, just for easier math, $1 per 30 ML and you cut 50/50 you would get a total cost of:
$11 per 30 ml, so your savings would be $9 per 30 ML of finished product.
I hope that is what you were asking about?