First? The cake in your photo was probably $17-20 per serving. Just for perspective. Next, did the price per serving include all the sugar flowers?
The prices your baker quoted you are almost spot-on to what I would charge (except I'm more expensive per serving based on where I live). I calculate the base price of the cake, which only includes a border and some basic decoration, then I estimate how long it will take me to do the design, multiply that by my hourly rate and charge it as an additional design fee. I charge $25/hour, and I'd probably estimate 4 hours to do that design (including flower placement). I also charge separate for sugar flowers, based on the sprays on that cake it would be another $100-200. Some cake studios just add both those things to the base price of the cake, so instead of charging 5/serving and then itemize the extras, they charge a flat 9.50 per serving and lump it all together.
If your baker isn't comfy attempting this in buttercream then find someone else because it can be done, but it will take some skill. It's simplicity makes is a hard cake to execute, especially if the bakery isn't used to working in modern buttercream.