They recycled a *LOT* of stuff. Even from other shows. For example some of the musical scores were made for Dragnet (another show that recycled actors). They also had to move pretty fast as they were filming one week, laying in the effects/audio reshoots the next week and showing it the next week. Most episodes took 2-3 weeks to make (unheard of at the time).
Most of their budget went for the special effects. A guy shooting a phaser could eat the budget quick as you had to have some one draw a cartoon on each frame.
At this time I can not justify buying the show again (got the DVD ver). I barely watch it anymore. Plus I filled my 20TB NAS recently so I do not have the space for a 3 year show on Bluray. I watched it quite enough when I was a kid