Having made a number of saddles myself, I would agree with your wife that the cloth/mesh and webbing can be purchased quite cheaply, BUT the buckles and rope used to build a saddle are more expensive. Check out the retail prices of 1” and 2” load bearing buckles like the Raptor or Cobra - generally in the $20-$35 range each and there are 3 of them on most commercial saddles. An amsteel bridge and prussiks will be another $15 retail, even a little more for Oplux and TRC.
Materials add up fast for a DIY saddle. Still not in the $200-300 range though!