alot, about 900.00, but I was able to sell my old case for 650 and I needed a longer front shaft so 4:1 gear set for the 300 was not going to help.
Are you talking about flipping the 300 or the 203? they clock the 203 all the time no prob, I think you might have oiling probs with the 300, but the seals should be fine, you would need to relocate the vent tube to the top of the output shaft and find a way to add/drain oil, as well as figure out how to shift the thing...prolly just better to buy the flip.
unless you need the length and or can machine the adapter/ shaft yourself, the doubler is kinda dumb.
think about it after you drop 750 into the adapter and the shaft, 200-300 into the 300, dunno how much for the flip...add 600 for 32 spline front and rear outputs...coulda just bought an atlas and shaved 100lbs at the same time...
Don't get me wrong the idea of low, lower and damn low is cool, plus I needed the longer front shaft and I got good chunk of change out of my old case...but there are easier ways to get 5:1.