There really is no “standard rate” for musicians and bands. The fee must be negotiated so that both sides feel they have gotten the best deal possible. There are times when a band plays for free, such as at a fundraiser for a worthy cause. However, there are costs involved with every performance, such as travel, rental of sound systems, costuming (or dry-cleaning stage performance clothing), food, and incidentals, so it’s normal for the band to get their own costs covered even when they are doing a performance gratis. I’m not in favor of playing for free when someone else is making money from your performing, like at a nightclub.
Most community organizations, schools, non-profits, corporations, and municipalities have some budget for allocation to live music at their events. It’s professional and smart to ask about the budget when you are asked to play. There are times where it might make sense to play for free to get “exposure” but this is actually really rare. Professional dance bands might make $5,000.00-10,000.00 a night (where I live). A DJ might make $1,500.00-2,500.00. The local musicians’ union posts musicians’ rates publicly, but these rates are often much lower than what bands actually can make.