When you put price in the field, MiniVend will do exactly what it says in the documentation;
multiply the value of the products
database field price by the number of items in the cart. If you have quantity discounds, price
adjustments, or other discounts in force via
CommonAdjust or another facility they will not be taken into account.
The solution is simple -- put [subtotal] instead of price in the criterion field. MiniVend will interpret
MML tags normally in that position, as it will in the cost field.