No, his value is not solely tied to his defense. The run values for batting, fielding, and position are all measured against average. So -.3 batting runs just means that he's been a smidge below average at the plate.
Even if he had, say, -5 batting runs and 0 fielding runs, he would still be 6 runs above replacement, and still have some value (about .6 WAR, which they'd estimate at around $2.6 or so).
It's hard to put a $ figure on one element of his game because players are paid for the overall package. If Crede was the best fielding third baseman in history, but was -30 runs with the bat, he would be worth about zero.
If he was an average fielding third baseman (0 runs), Fangraphs would estimate that he was worth 10 RAR, and about $4.4. So you could say that he is worth $6.5 more than he would be if he were an average fielder.
Of course, that is based on his actual performance this year, not his estimated true talent. He's almost certainly not "really" a +23 runs/150 games fielder.
Since his offense is a slight negative, does that mean he's essentially a "10 million dollar defender"?
I didn't think that was a tough concept.
I thought you were questioning it.
I get what you're asking now.
His defense is worth 11.9 runs which is worth just above 1 win, which is about $4.4 million. The definition of replacement player is roughly an average defender and well below average offensively. Since he's average offensively, a lot of his value comes from there as well (through the positional and replacement values). I don't know if that makes sense.