We've put a generation's worth of engineering into our printers and have a huge supply chain advantage so for any price point you pick it would be very hard to build a competitive laser printer, since printer manufacturers typically take a very large loss on each unit sold.
But that's not really the question here - on my homebrew projects I usually don't try to be cost competitive, it's about the learning, fun and pride in the finished project.
I think the main reason there aren't any open source printers is that the refill/counterfeit ink and toner markets somewhat solve the problem of expensive supplies - you already get better hardware than you can build yourself and you can get cheap non-brand toner/ink.