whats it take to convert my 21 spline np231 to a 23 spline? most importantly, how much does it COST?!

I'm either going with this t-case with a slip yoke eliminator or with an np208 form a dodge or jeep. my first estimate was 450 which I think is a little steep for the 208.
what do ya think?

You just need to switch the input gear, the price depends on the cost of the gear.

Be carefull, there are a few different tooth patterns on the input gears.