Which is why only the major publishers do physical releases in stores these days, the margin is very slim because the retailer needs to make money to keep their lights on. Also with the long lead time required to produce the disks whatever you ship is invariably out of date by the time it hits the store shelves.
Anyway back to digital distribution:
Steam is well known to take 30% after VAT/Sales Tax and whilst this can probably be negotiated with Valve if you have significant leverage* lets take it as a given. In Frontier's store the fees are closer to 4% before VAT/Sales Tax and although they will also have costs associated with running a store I'm pretty sure it won't affect the numbers that much.
The order in which the fees are deducted in does make a difference as the tax man only cares about receiving a set percentage of the retail price, the fees are not their concern. As such Frontier receive around 21% more per unit by selling on their own store than they would on Steam.
Steam may have a bigger reach but Frontier successfully sold ~419,000 copies of Elite: Dangerous via their own store before listing the game there so it's not insurmountable to do it on your own.
I do believe Frontier will launch Planet Coaster on Steam but they won't do so until the game releases outright at the very earliest. If we took the 419,000 figure for pre-Steam sales of Elite: Dangerous as an example they would be losing £1.76m ($2.57m) to Steam fees (at £19.99 pre-order price) and that's a lot of money that could be better utilized in paying developers to make the best theme park simulation game ever. [yesnod][up]
* I have long suspected Frontier cut a special deal on the fees for Elite: Dangerous due to the timing of it's surprise addition being just days after Valve announced they were sending Vive Dev Kits out to select developers. [alien]
Hmm ... This is not exactly how it works ...
I'm agree with you that 30% is pretty huge, but the thing to understand is that they have 50,000,000 registered members on the plaform and to be placed in a store with such striking force has a cost, it's what you pay in the 30% (that, I repeat, can be negociated, and truly. Not a single big studio pay this percentage, or they should dismiss their salesman lol)
So, until now, they generated 690,000 sales on Steam, sold at an average price of $44.82 (excluding discount sales period, so it's less than that, so let's say an average price of $35), that mean more than $24,000,000. Even if you subtract 30%, it's approx $16,000,000 earn by using SteamStore so far. And to use your own sentence "that's a lot of money that could be better utilized in paying developers to make the best theme park simulation game ever"
So one questions : Do they have gained as much using only their own sales platform (Frontierstore) ?
And these are the only ones able to answer, because it's clearly impossible to make predictions close to reality because the frontierstore stats are private, but for me, it's much more interesting to sell your game on your own platform AND SteamStore, including with 30% commission, AND 2 or 3 others online stores (to diversify a bit its distribution channel)
For me, they are still negotiating with Valve. [rolleyes]
Just my opinion.