If a .45 is really what you want, that's probably what you should get. If you get anything else, you're still going to want a .45 since that's what you originally had your heart set on.
But if ammo cost is truly a factor and you consider .45 a bit pricy, 9mm is the cheapest of the service calibers, followed by .40. Both of those will serve you well as defensive rounds.
Build a man a fire and he'll be warm for a day.
Set a man on fire and he'll be warm the rest of his life.