For "Reimbursement" based on your scenario; you only receive $30 reimbursement. If there is also a fee, that is additional. As mentioned already, some shops may offer a flat fee, what you spend determines your profits.
There are several companies that knowingly offer a flat fee or "reimbursement only" (or combination of both) that will not cover the minimum required purchase. A few offer coupons towards future meals with the client.
I recommend looking at your reason for being a mystery shopper as far as determining what works best for you, in each situation.
My posts are solely based on my opinions and for my entertainment, contact a professional if you need real advice.
When you get in debt you become a slave. - Andrew Jackson