Onsite PCI assessments are not cheap. First make certain that you have to comply with the onsite assessment requirement.
Although all of the major card brands are partners in PCI-DSS the number of transactions are counted by individual card brand.
For example, a merchant that processes 2 million credit card transactions will not necessarily be a Level 2 retailer. What matters for purposes of this requirement is the number of MasterCard transactions. You may have 800,000 MasterCard transactions, 600,000 Visa transactions, and 600,000 transactions with American Express.