Sportsmobile builds to your needs, and is generally not as expensive as buying one of the luxury trim Bs and then modifying it. But it would not be as inexpensive as starting with a cargo van, especially a good used van, and doing it yourself. There are a number of sources for the kinds of equipment going into conversion vans, and if you can't do cabinet work yourself, you can usually find a cabinet maker locally. Or see what from Ikea can be modified.
For cost purposes, it is important to figure out whether your goal is to have a motorhome, or a camping van. There is a big difference in initial cost. Not many B builders do camping vans. But if you want a bathroom, you are looking at a motorhome build, because of the supporting infrastucture, water and waste tanks, water heaters, electrical systems to power RV appliances, etc.
For a more basic B, look at the one Winnebago is building on the Ram Promaster. Not as luxurious as ERA, RoadTrek, Airstream, or most Pleasureway Bs, but still a lot more to it than a camping van.
For a commercial build of a camping van, look at the VW campers over the years, Westfalia and Sportmobile version, and Pleasureway's Traverse. Price gap between these and motorhome-like Bs has been $20k to $50K, over the years. Difference is being designed to support camping lifestyles versus design to be a complete, very small house on wheels.