LiveUpdate is the term we're using for a technique similar to downloadable content, it will not be a general-purpose backend solution. We understand that maintaining a large scale server operation is not ideal for indie developers, and we are putting a lot of effort into designing these kind of systems to be as easy as possible for people to use. We're looking at the default/basic use case to be uploading files to a server or hosting solution that are capable of serving these files over HTTP(S), no configuration specific to Defold required.
Since you mentioned S3 I took a look at their prices for cloud storage (which can be found at https://aws.amazon.com/s3/pricing/), I looked at their EU Ireland prices.
I'm looking at a worse-case scenario here where no optimization is done in terms of batching, and I'm also assuming that every user has to download all the content and ignoring the possibility to download new content as users progress through the game.
For a game with 500 different resources that requires 500 MiB storage, there would be a fixed cost of $0.0115 and a moving cost of $0.05 per new user.
For a game with 1000 different resources that requires 500 MiB storage, there would be a fixed cost of $0.0115 and a moving cost of $0.055 per new user.
For a game with 500 different resources that requires 1 GiB storage, there would be a fixed cost of $0.023 and a moving cost of $0.095 per new user.
These are of course very basic calculations of price without factoring in retention, major updates etc. A more in-depth analysis of the cost for a small, medium and large game would be very interesting if it would be possible to use numbers from an actual game for these types of variables.
Edit: Realized I mixed up the posts of @seepia and @Aiki while writing this...