Alien wasn't low budget, it quickly became an iconic film that still influences till this day in terms of the design, the monster and the heroine.
Universal gave Riddles $200m for Robin Hood and Fox gave him another $200m for Exodus.
Alien was a low-budget film. They keep saying it over and over again on the Anthology.
As for your second remark I stand corrected. So how come Ridley only got such a (relatively) small budget for Prometheus and Covenant?
Did Fox lose faith in him or the Alien-franchise just because Exodus and Robin Hood didn't do that well at the BO? Were they expecting the same succes as Gladiator?
Imo Ridley should have just stormed out of the gates, guns blazing when he made Prometheus. He only had one shot to get the franchise back on track but he blew it. Prometheus was obviously meant to be a set-up for future films.
Now we got bald humanoid bodybuilders for Space Jockeys and alien eggs that are simply the result of a mere experiment by some random runaway droid, contadicting everything we've seen in the series.
Congratulations Fox / Ridley! You definitely know what your fans want....