Cost for a basic mixing set up for weddings or parties? I might get back into this.

So a full live sound setup for bands or DJing?

Room size is important to consider.

Probably under $1k. You don't really need mixing as most sound platforms will have a built in mixer in the software. Your expense will be mostly the PA system and like someone said the size of the venue is a big factor. You can literally set up a playlist for itunes/spotify and take any requests to add in. Will need a couple mics for letting quests make speeches and for announcements. Probably pays for itself after a couple of gigs.