They don't have to be so expensive, it is just that the mainstream market in B's today is a luxury market, and that's what the dealers are ordering, since they are not having trouble selling them, even taking large margins. Most of what you see is way above any entry level C or travel trailer in cost of fixtures and trim materials used.
An extended-body 3/4 van today runs about $35,000, somewhat more for the Sprinter. A converter can put $30,000 into the conversion or $100,000 into the conversion. Winnebago's Travato is a medium price conversion on a low-cost van, their ERA is a premium conversion ($80K range) on a much more expensive van.
So for lower cost type B RVs today, functional but not so plush, look a Winnebago's Travato. It gets a bad rep among B reviewers from time to time for being so "plain" or "cheap."
Pleasureway used to sell a lower cost conversion on the E-series van, the pop-up roof Traverse. It was more of a camper van than a full-fledged motorhome. Roadtrek offers a Ranger RT on the Chevy van that is about $20K less than the similar-size Popular 190 or even $10K under the smaller Popular 170; Ranger is less lushly fitted out.
Sportsmobile routinely does conversions at the $30K-50K level, might build for something less if your needs are simple enough. An equivalent of the 1960's camper vans might be done DIY with about $10K worth of fittings and materials, but you are likely to put $20K worth of your own labor into it, if you value it as highly as labor costs in a RV conversion shop.
Look at the Travato (dealers will have them), try to find a Ranger RT (or at least look at the description), then check out Sportsmobile's web page, where you can sort of build what you want and estimate the cost (price).