Species is probably the wrong term. What I was saying is that beer yeast is very carefully maintained to avoid mutations in fermentation profile and bakers yeast isn't because any old yeast will do, it really survives more than a day at most.
Have you ever tried baking bread with any old yeast? Do you think that's what commercial bakers do? I bake bread once or twice a week, and to my knowledge the bread yeast manufacturers are just as careful about consistency as ale/wine yeast makers. But now that I'm thinking about it, there are quite a few differences:
1. Bread yeast has to act fast -- acceptable fermentation time is measured in hours, not days or weeks.
2. Bread yeast is intended to be used at somewhat higher temperatures than ale/wine yeast (room temperature or a bit higher -- no lagering)
3. Production of higher-order fusel alcohols is OK in bread making, because the heat of the oven will evaporate them away. Fusels in ale / wine / mead tend to linger, and negatively affect the taste.
4. Flocculation / clarity is utterly unimportant with bread yeast, not so with ale / wine / mead.
5. Bread yeast sees / expects a very nutrient-rich environment compared, to mead anyway (maybe less so for ale or wine). You never hear bakers talk about nutrient schedules or organic vs non-organic nitrogen, etc.
6. Bread yeast doesn't have to contend with potentially toxic levels of alcohol. It does produce some alcohol, but the alcohol level never raises to 5 or 10% of the surrounding environment like ale / wine / mead does.
7. Oxygenation is probably different -- I don't know how much O2 is available inside a bowl of dough, but I suspect that it's different from the levels you'd get in must or wort.
I believe that the two kinds of yeast are both carefully controlled and optimized for their own respective environments. Even so, some people have had success using bread yeast to make alcoholic beverages, and ale / wine / mead yeast can work for bread-making (I have a starter in my fridge right now, made from ale trub, and it's been producing good bread). Which makes sense too, because in days gone by (medieval times), bakers obtained their bread yeast by skimming yeast from the surface of fermenting ale. But the two types have diverged since it became a commercial product, both of them being optimized for their own particular intended usage.
So maybe the answer to my question about why is bread yeast cheaper, is simply that it's easier to consistently mass-produce those strains of yeast that can operate successfully in a baking environment vs a brewing environment.