Alex Adventurer rims = $60/pair
DT 14ga spokes = $57/box of 100*
XT 770 Hubs = $125/pair
*assume similar price to purchase smaller quantities of 3 different lengths (2 for rear, 1 for front).
Total = $240 in parts, another $40 - $50 each if you pay a shop to build the wheels, so at least $300 for the pair.
You mention a sturdy wheelset for $250... What's the build on those?