Here is the answer I recently gave someone:
It has nothing to do with its power of effectiveness although it is a very powerful attunement. It is because of what I have to go through to get a hold of some of the manuals and information for the attunement. I can attune someone to anything however it is important that I be able to give the attunement manuals as well. With Lavender Reiki I really had to go through a lot to get the manual including dealing with many other Reiki Masters that were very much against the way I have chosen to do things, such as giving the manuals for free. Right now I think the least expensive Lavender Reiki attunement I saw offered was $25 and all the others were $45 and $50.