It depends. Some shops offer only a reimbursement for purchases (common for restaurant shops), some offer a payment plus reimbursement (common for many retail assignments), and many shops are rewarded even though the shopper is not required to spend ANY money out of pocket (common for bank and apartment shops).
Payments will often vary depending on an array of factors (e.g. the complexity of the work, standard of living based on United States geography, availability of shoppers in a geographical area, etc.)
Shoppers are ALWAYS informed of the payment and expense limit of a given assignment. Furthermore, as an independent contractor, you are always at liberty to accept or decline any assignment.