Roger Corman is/was the greatest B-movie impresario of all time. While his movies were low budget he never purposefully intended to make a bad one. This is why his films are usually pretty decently entertaining. His Poe films and such are all quite good and all on minimal budgets. Corman could really squeeze a nickel.
Movie makers should be required to view his work to learn that decent films can be made without massive budgets. I believe that, in general, the bigger the budget, the worse the SciFi movie. Now, I know there are exceptions. Yet so many of the best films are done with relatively lower budgets. Star Wars, Alien, District 9, Terminator (original), Pitch Black, Primer for example. Versus anything by Michael Bay or (God help us) Avatar, or the last three Star Wars flicks.
I will add “Forbidden Planet” to your list of lower budget sci-fi films that were very good.