Student Bodies might have been the first horror comedy/spoof. It's definitely the cheesiest. "Horsehead bookends!" Dark Star is available on blu-ray, though not a huge improvement over the DVD. Student Bodies is out there on DVD, though possibly hard to find.
Maximum Overdrive - $9 million Invasion from Inner Earth - budget unknown, but it had to be pretty small
Pretty sure Abbott & Costello Meet Frankenstein beat it to the punch. Not to mention The Cat and the Canary, The Ghost Breakers (with Bob Hope), The Ghost and Mr. Chicken, etc. Meanwhile, my favorite low-budget sf movie is probably TIME AFTER TIME.
Was that the one where they were trying to go inside the earth using lasers and what looked like mine cars?
I'm arbitrarily limiting my selection to some non-fan-film sci-fi films, with a budget less than $1M (unadjusted), made after 1970, that I've seen and like enough to watch again. Split (1989) Dark Star Primer Cube THX-1138 Silent Running Phase IV (pretty sure this has a low enough budget to qualify)
Outside of Star Wars, two of my top three movies have been mentioned already - Terminator and Silent Running. The third one's Aliens, which I don't think was low budget...
The Ice Pirates Cheesy 80s space action adventure movie with more or less mediocre acting but awesome one liners and robots with swords fighting each other. It's about a group of Space Pirates who are stealing Water.. the most precious resource in the galaxy since apparently all planets are desert planets and everybody forgot how to get to Earth (and never mind that Hydrogen and Oxygen are amongst the most common elements in the universe). Sprinkle in some Alien type creature, time travel, Space Amazons and a love story straight out of a supermarket romance novel and you got the movie.. it's awesome!
Virtual Obsession which felt a lot like a made for TV version of Transcendence. Made in the 90s... http://www.imdb.com/title/tt0144239/combined My long winded review is here too http://www.imdb.com/title/tt0144239/reviews
I'm not sure that's a spoof as much as a straight comedy. Maybe not a huge difference. Student Bodies, like the later Scream movies, made a point of making fun of horror movie tropes. A&C Meet Frankenstein was just plain funny.
http://www.imdb.com/title/tt0167720/combined 7 Days........ I just wish they'd sorted out the rights mess as to who owns the rights.
You could argue that A&C Meet Frankenstein were spoofing the conventions of the previous Universal horror pictures of the time, right down to casting Bela Lugosi and Lon Chaney Jr.. Ditto A&C Meet the Mummy, The Invisible Man, Dr. Jekyll & Mr. Hyde, etc. Basically I was just making the point that horror-comedies are a grand old Hollywood tradition, including innumerable "old dark house" comedies like the umpteen versions of THE CAT AND THE CANARY. Heck, THE GHOST BREAKERS with Bob Hope was at least the third film version of the original stage play--and was later remade with Jerry Lewis and Dean Martin. And those movies were spoofing the cliches of previous thrillers: secret passageways, sinister servants, ghosts, zombies, thunder and lightning, etc. https://en.wikipedia.org/wiki/The_Ghost_Breakers
The original Alien (1979) had a budget of $9-11 million. The sequel Aliens (1986) had a budget of $17-18 million.
A group of young pilots in a remote region of the Canadian wilderness begin to hear strange reports over their radios about planes crashing, cars stalling and a deadly plague which has gripped the planet. It becomes clear that earth is in the midst of an invasion. The group of pilots decide to barracade themselves in a cabin deep in the woods and wait for their impending doom.