I get the budget would be lower and could that have effected the featured cast like Nichelle Nichols and Walter Keonig?
Well, per contract, NBC's license fee would have gone up if a hypothetical fourth season was made, so that would have closed some of the gap.
On the other hand, per contract, all of the show's regulars (at that point, Shatner, Nimoy, Kelley, and Doohan) were entitled to raises if their options were picked up. Takei, Koenig, and Nichols had agreements that were less black and white, but they probably would have gotten small raises if they came back, too.