The butterfly pattern is elusive because it is variably expressible. Meaning that if you breed two bettas carrying the BF genes, you should (in theory) get some BF in your spawn. How many of them and how perfect of a pattern is simple odds. Impossible to predict. Some will have just a white or clear line (or red or black depending on the BF color), and some will have fins that are more 50/50 (most desirable). Some will have the 50/50 color on all fins (best) and some will have it on one or two. Further more BF pattern may change overtime, so some bettas may start solid and turn into BF at a later age (my Apaches do that oftentimes), while other may start as BF and then turn into marbles. 

In short, perfect BF fish that have all three fins bi-color with a 50/50 color balance (as in half of the fin is one color, the rest is another) remain very hard to produce, hence their higher price.

If you spawn two solid bettas that do NOT carry BF, your chances of getting BF bettas are slim to none. So to get BF you MUST start with a pair that carries the gene. Another way to get some is to cross a solid betta to a marble. The BF gene after all, does come from marble. Confused? Don't worry, in time it will all make sense LOL.