Is it expensive to rent audio equipment?

No, Not at all. Also, it may be more cost effective to rent sound equipment for a longer period, perhaps a few days rather than just one.

It depends on what type of audio equipment you want to rent. For light audio setup it will cost low and for heavy setup it will cost high.

No, It's not good. Buy it if you need it.

sometimes it is good to rent audio equipment but if you need for long time use then buy that permanently.

Not at all. Check this site for example: https://www.accordequips.com/rental_equipments/audio-equipments-on-rent/