No idea about potential salaries but I view both Mills & Collison as better fits due to their range & ability to play off the ball. Rubio isn't a bad option based on age/possible remaining potential but I wouldn't give up much, if anything, of value to acquire him as I feel he would clog up our offense because of his limited range & ineffectiveness without the ball.
As far as salary dumps go, Jeremy Lin would also seemingly be a better fit &, like Rubio, has 2 years remaining (assuming he accepts his player opt) at less money (2/24). But I would pretty much be okay with any stop-gap PG who doesn't require a long-term commitment & keeps us competitive enough to entice Hayward to re-sign.