At the time Avatar was made I came to believe that the more $ was spent on producing a SciFi movie, the worse it was. At that time Dsitrict 9 was released, comparatively low budget, and kicked the crap out of Avatar. Yes Avatar’s effects were great but the story was puerile garbage aimed at 12 year old intellects. Some of the best SciFi has been low budget. Terminator. Alien. Even the original Star Wars had a budget in line with most movies and not exceptional at the time.
Crap like Godzilla vs Kong or Transformers have huge budgets. And suck.
A micro-budget sci-fi you might enjoy:
Sound of My Voice
2011
R
1h 25m
https://www.imdb.com/title/tt1748207/?ref_=fn_al_tt_1
Other successful low budgets: Predator 1 and 2; Forbidden Planet; The Thing (original).