As noted, a BG-sized piece of ebony- or even rosewood- isn't cheap. BGs can get by with a 1/8" slice; a UB fingerboard is a lot thicker, wider and longer. The Azola baby bass is one of the simplest EUBs around, at least from a design standpoint, but you still have some massive slabs of wood there.
It takes a certain amount of space- which has to be purchased or leased, insured, heated, and equipped whether you're making one bass a month or 100. You need staff, advertising, sales, and so on. And the kind of automation needed to make those $250 P-Basses that Fender sells today requires millions in up front capital investiment.
There's a general rule of thumb that says manufactured items need to sell for about 5x the marginal cost of production- so a $1,500 EUB might cost $300 to make before you add insurance, marketing, rent, and all those other factors. Could you build one for $300 in your home shop?