Depends entirely on what you're selling, the margins, the conversion rate of their landing page, and conversion rate of their sales funnel. Some companies are on auto pilot and are over paying, but the math and techniques are simple for SEM ROI when done with a little attention.

If what you're selling is mortgages or franchise businesses the cost you are willing to pay for a click just to get a chance to convert to a lead, just to get a chance to convert to revenue, is surprisingly high.