I was more often on the giving end of live music at parties than on the receiving end. Most of the time we’d just play for free or for beer. It depends on the function. Weddings, frat parties, you charge a lot. Friends’ parties you play for the fun of it.
If you’re hiring a band, I guess you’d ask around to see who the good bands are, what they play, etc. I suppose a couple hundred bucks would be in order for a basic four piece band. If one of your friends knows someone in a band, you might be able to get them for cheap.
You’re right about not wanting to bother the guests or the band about gathering tips. If you’re lucky some friends will probably offer to chip in for the band or other expenses.
About the balance of men & women thing - you don’t necessarily need a one to one ratio. It’s just that having women there - even if they’re already spoken for - uplifts (to varying degrees) the party from being just a bunch of guys getting hammered. If you’re gonna have live music or dance music you gotta have someone to dance with!