That seems like an interesting product, Victor. But it's not something many people would be able to use. Unless you find a partner capable of incorporating these bases into a finished machine, I think it will be difficult to find enough home-builders of large milling machines to support your production. The question isn't really whether most of us here wouldn't rather buy a pre-made base than make one; it's whether we can afford to do it. Many people on this site, who are probably representative of home-shop machine builders in general, are trying to build what would be very expensive machines very cheaply. Even if your bases were less expensive than conventional cast-iron bases, it seems doubtful that they'd really be cheap enough for the hobbyists here.
Rather than trying to sell them as components, I'd suggest looking for a company that can fit these bases with the rest of the parts that would make them into complete milling machines or machining centers that would meet the specifications of industrial customers, and could be sold as such. Either that, or source the parts and systems yourself, and set up your own company to market them. If they really had a significant price/performance advantage over the mills that are currently produced here (or, more commonly, imported) it seems you'd be able to sell them at a profit, and in sufficient quantities. I'm sure you'll be able to get suggestions here as to what would be desirable in a mid-size mill; which of them you decide to adopt would be up to you.