Most affordable way out on the water for the day is to do an organized trip. These are called join-ins and average about $160/pp. Check out Bradley Powerboats as well as DiveBVI for itineraries and options. You can view different trips on both websites. These are a great way to see multiple popular spots (Willy T, Indians, Soggy Dollar, etc). Both are great companies.
Fully private day charters allow you to customize your day but are going to start at over 1k regardless of how many people you are. Plenty of good options for who to use.
Let us know which way you lean so we can help you either pick which join in fits your wants or which companies you may want to inquire about private hire.