If you are talking budget, STIII has a planet, falling apart, with fire and breakaway sets, I don't remember THAT in STII. Also, besides the Genesis cave in II, there are NO other 'exteriors' required, smart for a film to be shot on a TV movie budget. That is not the case for STIII. Nimoy and crew had to simulate 'all varieties of land and weather' on a modest budget. STIII also has more new ship designs, like them or not.
ST IV was a masterpiece and the best thing to happen to Star Trek since the second season. It brought the franchise to a whole new mainstream audience and made a ton of money, and may very well could be the reason Paramount green lit STTNG.
How anyone can dismiss Nimoy's directing contributions to Star Trek is beyond me. Without him we wouldn't have had the same ST IV or VI, THAT is a pretty good track record, not to mention other bankable film success. Three men and a baby is a classic
PS. ST V stinks, and you know it