My guess would be that that someone who was booking a quartet   for a banquet would expect to pay a minimum would be about $700-800 per hour exclusive of travel and sound system expenses,-that would just be background music, not a show--and it isn't uncommon for a show act to ask $5000-10,000. You have to remember that corporate banquet events are big ticket items--food alone often starts at $100-150 per person--even a relatively small dinner might be $15,000 or more, and, depending on size, $200,000 events aren't unusual. And big events may have several bands.

It will help a lot to find out how many people will attend the event, and whether it is a stand-alone event or part of a conference or convention. Your sound person should get the same cut of the take as a band member--for events like this, professional sound can mean the difference between success and failure, and the sound guy should scope out the space ahead of time, and the band should do a rehearsal with him to set levels, etc.

You charge more for these events because it is actually a lot of work to do them right.

PS--Very bad to have two people quoting two different prices--one and only one of you should handle bookings.