Depending on size of your group, you can work with a cruiseline's group tours agent to arrange a trip. I'm going with a group of fifty, and we've got a block of rooms together, on board perks & discounts.
It's hard to nail down costs, because there are so many variables. Cruiseline, duration, ports, time of year, cabins, booze (and more) all affect the price you wind up paying. As a veteran of 10 cruises across various lines and also international travel via flying, resort staying and beach house rental with big family groups, cruising is best value and least hassle. Yes, backpacking, youth hostel, Eurailpass, camping and couch surfing or AirBNB are all "cheaper" in many respects, but with an 80 year old & a couple or tots, cruising is a great idea.
Carnival is the most budget conscious line, in my experience. Avoiding the spring-break party crowd is as easy as choosing a 7 day instead of a 4 day trip. If you're really cost conscious and they go to the ports you want to see, dive in, it's a good intro. Also, a good travel agent is worth their weight in gold.