This idea is only based on economics. First in general males are worth less than females so it makes sense to buy the cheaper of the two when it comes to high priced morphs. Secondly males can be bred to multiple females so it again make sense to have them in more higher priced genes to better spread their desirable genes. Males also get to a breeding age sooner which means a faster return on your investment.
It is not that you shouldn't breed a less expensive male with a more expensive female, It is more that it makes the most economic sense to own many low priced females and a few high priced males. Of course this will not always fit your breeding plans, and you will have to breed a high priced female to a lower priced male to complete your goal. This may make economic sense if it puts you in a place to make a higher priced morphs in later generations.