I imagine the reason that chassis builders are reluctant to give a price is that they never know exactly what the customers' demands are going to be when they start a chassis.
For example, a price might be agreed for a " basic rolling chassis" but that means different things to different people. Then you get the extra requests like: "Can you just add me a radiator?" or "Can you just do me an exhaust system?" or "I've changed my mind, can you mount the fairing, wheel arch and seat unit?"
Building a chassis is as much about time as components. It's relatively easy to price the components but virtually impossible to know how much time will be spent on doing the extras that some customers expect for no extra cost.
The basic chassis comes together fairly easily; it's all the brackets and ancilliaries that take the time.
Another thing to bear in mind is that would-be customers are often not comparing like-with-like.
Racing sidecar outfits are usually bespoke items, built to customers' individual specifications. However, I assume that Louis Christian has enough customers to be able to have more of a production-line system for making outfits than most others. I would also guess that most other chassis builders don't make enough outfits in a year to gear up to make batches of parts. Time spent making , say, five sets of forks is time spent NOT making the rest of the chassis (and the customer is breathing down your neck, demanding you finish it quickly!).
The main point, as Mike says, is to make sure that you agree exactly what you are getting for the stated price.
(It reminds me of the old joke: What were Nelson Mandela's first words when he was released from prison after 27 years?
"Has Rod Bellas finished my chassis yet?"

).