I did my math once a while back, i think for the first year it's not that expensive, it could increase depending on what you setup if you are not careful.
Could go from a few dollars(or less) a year to a lot more.
I think the real "cost" would be in your dev time, or in other ppl doing it for you, setting things up.
You would use, S3 for the files, incognito for signups/saveFiles sync and DynamoDB for the db setups.
Dynamo is surprisingly basically free for startups and small studios(low-end consumption), incognito is pretty much the same i think, and S3 is potentially "inexpensive". That's going to be a relative term based on how much your game made vs how much you used.
Setting things up is not necessarily simple. I looked up their unity integration in the past, it seemed ok, but this is something they made themselves not third party.
Frankly just the sheer responsibility of accounts(personal information), sign-ups, handling downloads and server times, etc. Will frighten a lot of ppl away form this, hence why built-in solutions are a better solution. Also most "services" that pretend to handle this(back-end), offer way much more than simply "dlcs" and co, and therefore charge a lot more also.
I think if you guys set this up in an easy built-in manner, where gamers would login with their "King(Defold) Accounts" for all games made with Defold, it could work.
Even if you guys charged a very small amount like a few dollars or whatnot an year you could probably make some money off of the storage, not sure about the maintenance costs.
I mean how much is the most advanced and bloated Defold game going to use? 1GB, 500mb? You could max the storage at that. The key thing is how much all the maintenance will cost for you guys. Unity offers unlimited cloud builds, which is potentially more expensive and complex, for free, but they are selling a product, so they have the cash.
Idk really. Someone who knows a lot more than I do has to sit down and really add the numbers, but maybe there is a system that works for you guys and for everyone else.
Either way, seems like native extensions will be a solution for integrating third party services and apis, for ppl who want/need them.
I wish you guys success!