Well if some of you want to pay <$100/£100 for a theoretical S3 then sure, go ahead.
We can only assume that S3 will bring to S2 what S2 bought to S1... if you follow my trail.
Assuming the current trend of £12 per stage, it's pretty reasonable for a 3 man team, that has quite slow updates (tbh.).
Say that S3 is released for $100/£100. Who the fudge would pay that outside of the die-hard LFS userbase? NO-ONE!
£36 is probably just about as high as LFS will get before people start to say "Hang on a minute, is this worth it?"
If LFS went above the price of most console/major PC games (£39.99) I'd seriously reconsider buying any additional licences.
I'd guess that buying S3 outright would cost about £30 (being reasonable for newcomers) and it would cost the standard £12 to current users. Which is fair enough imo.