In respect to your first comment as far as prices not going down for you, one probability is that you were booked on Royal Caribbean, Costa is owned by Carnival. Really no incentive for RCI to cut any deals.
As far as your second question:
If you are willing and able to put up the deposit, there is nothing wrong with booking early, sometimes you get the early deals which can be pretty good, but if you don't then you have plenty of time to keep an eye on the pricing. If you have booked through a travel agent, it is important to make sure it is one that will really work for you by keeping an eye on your booking. This includes special pricing, price drops, upgrades, on board credit incentives. Some are only available for new bookings but many are retroactive to an existing booking or an agent that has a good relation with their rep can get things working, NOTHING more valuable to a cruiser than a good concerned Travel Agent. BUT keep in mind if you book through an agent it can be almost impossible to fire that agent, you will be stuck with them, at least for this cruise, then lesson learned you move on, it can be trial and error.
Now if you are doing your own booking through the cruise line or with an online agency that doesn't provide a lot of support, (you will also be stuck with them for the current cruise as well) it will be up to you to do the dirty work but if you stick with it and are diligent it can pay off for you. You need to be on the cruise lines website VERY regular, checking pricing for your category of booking, upgrade offers, discounts, onboard credits, anything that will reflect a savings or improvement in your onboard experience. When you see something, you call, be it a travel agency, the cruise line booking department if you booked through them, and go for the throat, politely but the throat non the less, they are making an offer, you want it if it saves you money.
Another thing in your situation is to compare, a cabin with 2 adults and 2 teens can get real small in a hurry unless it is a suite of sorts. Compare the cost of a suite vs. an outside with balcony and an inside right across the hall. Most all cruise lines will require an adult in a cabin but that is an easy workaround, one of you books the outside balcony with one of the teens, and the other of you books the inside with the other teen, after you move in, you two get the balcony and the teens get the inside, the cabin stewards really don't care and sometimes it can be cheaper than a larger suite to accommodate the 4 of you, just compare and see what works best for you.
