Visa processed ~4.2T of payments last year (page 32) and earned ~5B of net income on ~11.7B of revenue (page 29). By my calculations that is ~0.1% off the top of the transaction net of costs to do business. Not much.

All from the most recent 10K: http://investor.visa.com/phoenix.zhtml?c=215693&p=irol-SECTe...