While it's different people, you'd have thought The Final Frontier would've had a bigger budget based on the financial success of the prior four films. Paramount may not let another director draw up a huge budget for a follow-up that isn't done by Abrams.
Watching the films now, it's hard to believe, but the studio did give Shatner more money than Nimoy had on the fourth film.
Nimoy's movie cost the studio $22 million to produce (it came under its $23 million budget). Shatner's movie cost the studio $32 million (going over its $31.25 million budget).