I'm not too sure about your arithmetic. On the back side, every cow you retain has a 9 month cost to keeping her. How would the heifer be any different during those 9 months?
The only added expense I see is the time and money invested between birth and breeding... which is canceled out depending on how much a replacement animal costs. Those 15 months may or may not be comparable.
Granted heifers have their own issues, but some people manage theirs to succeed where others haven't figured it out. I'd certainly rather calve out heifers I raised and bred myself rather than buying younger bred animals that I know very little about.