The company I work for does custom high-end cabinetry. We make all our own laminate tops and install all our own products.
We do not produce our own Corian, but work with another company. We add a modest profit and leave it at that, because of the "hassle-factor". Our company's opinion is that the hassle it might cause in "call backs" and added on-site time would take away the profits.
I believe we can greatly increase our profits by doing the solid surface in-house. We have 30 employees. I think we have most of the equipment, except a V-groove machine. I need help to factor in production time, material costs, time to install and the all-important "hassle-factor". I am more of a bean-counter than a production person.
With 30 employees, it's worthwhile to get several of them trained and certified in solid surface fabrication. You can lower your price to the client while increasing your margin. The lower end price will also increase your sales of solid surface products. Don't get locked into just one brand. I find that color is the single most deciding factor in our solid surface sales (regardless of brand - Avonite, Corian, Surell, etc.). And your current fabricator might just cut you a better deal "to not go in-house". Either way, you have more to gain than lose. Hassles? You'll be paid for them - that's business.
Before going any further, find out if the local suppliers are even willing to certify your shop to fabricate solid surface. In our markets, the major brands limit who can do what to protect their existing customers. This makes it difficult to push a customer to use one brand when they really want another brand that you can't get. Do some background checking for equipment and supplies; otherwise you may very well be better off dealing with a shop that specializes in solid surface and putting your energy into continued improvement on the cabinet end.
Unless you do not have a good fabrication source in your area, getting started could be more trouble than it is (monetarily) worth. Consider what your gross Corian® sales will be, figure about 40% material costs, 20% direct labor and your overhead costs. What is left is your profit. Is it worth it or can you do better by tweaking your existing business?