People are assuming that, since S1 cost £12, and then S2 cost an extra £12, that S3 will again be £12.
That *may or may not* be true, but either way it hasn't been stated by the developers. It's purely an assumption by the community.
Though it seems like a reasonable assumption, it isn't reasonable at all because we don't know what S3 will contain, what it will be capable of, what WE will be able to do with it etc. Given this, it's actually pretty daft to state that it will cost £12. It's daft to even bother guessing at a number. The only thing we can safely assume is that S3, when it comes, will be purposefully priced to offer good value for money - one thing that is certainly a constant in LFS.
As for the release of S3, nobody knows. In the community we're guessing (and it's purely guesswork) that S2 is going to be another year or two before it's complete. Once S2 is complete, we assume that then, and only then, the devs will begin work on S3.