In my opinion, the only way to know if a price is "good" is to know what the standard offering price for a certain itinerary on a certain line is. Track the lines/itineraries you're interested in (at least to have a general idea). [what I mean is the standard price in a TA or cruiseline email, not the brochure price - as noted, no one pays the brochure rate ;-)]

On the other hand, sometimes those 3/4 passenger cruise free rates can be very good deals for families. Or so I'm told -we don't have kids. But in certain cases I've seen (I've tracked HAL in the past) those are actually good deals.