Low cost 30A 24Vto12V stepdown converter
While I'm not 100% sure on cost, a buck converter would very likely be the best solution for your problem.

As for your circuit working, it would work right up until your Vout reached 12V; then the MOSFET switch should open, but all the stored energy in the inductor would continue charging the capacitor to some overvoltage level determined by inductor/capacitor size ratios (and instantaneous load).  

Now, with a sufficiently small inductor value, a small shunt resistor and sufficiently high capacitor value, that could end up being a small enough difference to not matter. Depending, of course, on what error factor you're willing to accept in your 12VDC output.

For a better solution than trying to brute-force your circuit with a self-oscillating converter, the TL494 controller looks like it'd work well for you (using appropriately high-power MOSFET(s) for the switching, of course), and it's incredibly inexpensive. :)

A read-through of the datasheet for that controller should tell you what you'll need for how to set up your converter based around it.