Bob, I'm not sure about that particular set, but adjustable dumbbells would be a handy tool in P90X. There are a variety of exercises on Shoulders/Arms and Back/Bicep days that will cycle through a series of different weights. It's a pain having to manually adjust my dumbbells, especially when the interval between exercise sets is so short, like when doing strip sets. I would definitely recommend adjustables. You may do better than $300 for a set, though
Not sure why Bowflex is so $$$.
Depending on the lowest starting weight of that dumbbell, you may also want to pick up some lighter, single weights. You won't want to go too heavy on certain shoulder & triceps exercises (like Arm Circles) so make sure you can get to 3, 5, 10, 12 & 15 pound weights as well.
Good luck!