Agency fails to anticipate that computer scientist can do math when extending offer

Dudes I contract for-

Since I started working for you, my experience has increased by 50% (measured in strict calender time) and by, conservatively measured, a fuckload in actually accomplishing things. So a 4% raise for my next contract is not going to cut it- not when, under any reasonable set of assumptions, that works out to less than the permanent position I was offered, and especially not when the increase in commute amounts to a per hour pay cut.

And no, [large company] is not planning on paying x. They're planning on paying x+25 or so to you, which based on your desired profit margins works out to x. Do not blame [large company], because I happen to know exactly how much you are paid for me now and what market rate is for corp-to-corp is.

It was someone other than my regular recruiter who made this invitation to interview (the normal one apparently had an off-site meeting all day). And while I'm not saying they deliberately set up a bad cop to let the familiar good cop be a hero and offer me more, I am saying that's exactly what's going to happen.