Some of the more common ones aren't that expensive, Leslie. I don't grow them commercially so this is just my opinion of what I imagine the reason is, of course!
So most herbaceous peonies are propagated by root division. Most grow fast enough that they can be divided every 3-5 years and, under good growing conditions, probably produce a minimum of 3 saleable divisions each, probably more. The ones that produce more are typically the lowest priced herbaceous roots unless they're a really unusual color, new to commercial sales, etc. Tree peonies are typically propagated by grafting onto herbaceous peony roots--a much more labor and time intensive process. Tree peonies also mature more slowly than herbaceous so the wood necessary to create those grafts are more precious. The time investment alone in getting a tree peony large enough to donate multiple pieces for division is hugely longer than that invested in herbaceous propagation.
You can buy a bagged, tiny piece of the very most common herbaceous peony for around $5 but that piece won't produce flowers (probably!) for 3 or more years. That same variety potted and growing from a nursery with buds or blooms would be 5 times that amount. If you buy a root of that same type online the price will probably be $15-20, will probably have 3-5 eyes and at least one good sized feeder root and could bloom the spring after planting. The root necessary to produce a good tree peony graft would be that feeder root from the $15-20 herbaceous peony. Then the grower has to keep that graft going for a year or more before they can sell it. If you look around a 1 year old tree peony division can be anywhere from around $20 to over $100 depending on seller and variety.
There are plenty of unscrupulous sellers out there that will sell you what's supposed to be an expensive tree peony that ends up being a much more common, cheap variety once it blooms. The same thing for Itohs--you can pay top dollar for what's supposed to be a rare variety and end up with a common herbaceous. If you're new to peony buying it's worthwhile to do your homework and buy from reputable growers so you get what you pay for. You'll see factual reviews of sellers here on this forum. We even have picture of the roots we received for the last several years--if that's not good info for a new gardener I don't know what is! There are some reviews of tree peony growers as well, they just aren't as common as herbaceous growers. Most big growers at least offer some tree and Itoh peonies along with their herbaceous stock and are fine to buy from.
When you get your first tree peony bloom and it's actually the variety you wanted, selected because of a few pictures online, you'll see that it's worth the extra money for these amazing plants! The flowers in person are so much more wonderful than any picture can convey. And that tree peony is a longer than a lifetime investment with a lifespan as great as any ornamental tree that you place in your yard. I've seen plenty of posts on the peony forums where people have inherited both herbaceous and tree peonies from their parents, an amazing gardening legacy, don't you think?
