"Saving money" often turns out to be an expensive choice.
Compared to the costs of digging the ditch, running your circuits, etc., the cost of feeder wire is quite small.
At least run your wire in pipe- pipe at least one size larger than you need- so you can replace it easily if you ever need to. If you fail to run in pipe, or fail to bury it two feet down (minimum), my experience tells me you will be replacing it after a few years. Why dig twice?
Two ground rods? Why? One is plenty- though it would have been better to have placed a "UFER" in the slab. Oh, well, hindsight and all that!
My practice -and please note that there is room in the code for a few different approaches here- is to:
-Run a ground wire from the "main" panel to this "sub" panel, landing it on a ground buss (separate from the neutral buss); and,
-Plant the rod, and connect the wire from it to this same ground buss.
Of course, I make sure the neutral buss is NOT bonded to the case.
