The flat rate "game board" box is $58, and it is good for up to something like 10 pounds (maybe 20).
Most keyboards that I have shipped, well packed, weigh 5-8 pounds. Most carriers use a combination of weight and box dimensions (total of the 3) in some voodoo formula to come up with a price.
I would guess that if you packed it in plain cardboard, not flat rate, in a box that was not too much too big, it would go for $30-40 via Post Office.
I doubt seriously that you will get it under $30, and US Post Office will be your best bet.