Of course, the first thing I would say is it really isn't possible for salaries to be perfectly in line with a player ranking because the market is changing all the time, and every QB doesn't get a new contract in the same year under the same market conditions.
That said, the QBs that are clearly and unquestionably (in my mind) above Dak (not necessarily in order) are Brady, Brees, Rodgers, Mahomes and Roethlesberger, and ones that I also believe are above Dak, but by a lesser margin, are Rivers, Wilson, Ryan, Goff and Luck. From that point on, I think a reasonable person could put Dak, Wentz, Cousins, Watson, Stafford and Newton in almost any order, so that puts Dak anywhere from 10-16, depending on how a person chooses to rank.
Of course, Mahomes and Goff are still on their rookie contracts. When Majomes gets a new one it may be at $40MM/year. And that goes back to what I mentioned above, that salary can't really be looked on as a QB ranking. Whatever contract Dak signs will put him in a position on the QB pay scale that would be different than if every starting QB were negotiating a new contract at the same time under the same market conditions. The same holds true of the contract Cousins and Garoppollo signed. Their spot on the QB pay scale is not commensurate with where their spot would be in a QB ranking, but the market was willing to pay the price at the time they were negotiating.