you can get cheap .45 ammo if you look for it. Have you looked at Georgia arms, Cheaper than dirt and some other sites out there have it very reasonable per round.
It is all about what you want to shoot and what you shoot well. If you do not shoot your 21 well shoot your 9mm. It doesn't matter how powerful the round it if you are not hitting the target with it. I shoot a glock 20 and because of the recoil i tend to shoot slower than I did with my 9mm or Glock 21 45 auto. If you cannot shoot well with a gun you will not shoot it. I would love to argue about the ballistics but it has been done thousands of times. You can always make a 9mm deadlier it is hard to make a .45 more accessible and safer.
ga arms is 34.00/100. maybe thats cheap to you.