Wow, 15k for 4 people seems a lot!
Have you considered going to California Disneyland instead?
We've been there several times, done independantly it is much cheaper, although obviously the flight is longer.
If your ds wants to do Harry Potter than you will have to go to Florida. But California also has Universal, Disneyland, California Adventure (great for older kids), Knotts berry farm (if you like massive coasters), and 6 flags.
We've also done Florida, once in my twenties and once with ds a couple of years ago. They are different, even the same rides aren't always the same at the different parks eg pirates of the caribean is California is fab, Florida is good but not the same, and Paris is rubbish compared to the others and I wouldn't go there again.
Would definitely choose California over Florida next time. Florida was hugely more expensive, the parks were heaving every day, and we did a package which left me feeling ripped off by all the 'extras' such as flight meals, luggage, and the 'free' car hire which was anything but. Won't do a package again, but I know some find that easier.
In California you get less uk tourists so the parks can be much quieter during the week. The exception being Halloween. You can get a good neighbour hotel right by the park entrance so you can easily come and go, and see the fireworks from your hotel too even. We would go early, head back for a midday sleep and go back at night as we simply had to leave the park and cross the road. We actually had a longer walk when we stayed in an official disney hotel.
If you go for a hotel be sure to book somewhere that includes breakfast as lots of places don't. Also check if they have a fridge so you can stock up on drinks and food. On the plus side they don't take the payment till you arrive so you don't have to pay months up front and it gives you more time to save up, and you can cancel up to the day before. I check BA website for flights and wait to buy them in the sale. You can easily see if going one or two days differently is cheaper on their website. Then when you have the cheapest dates check other airlines to see if their tickets are any cheaper.
Disney hotels aren't cheap and you can spend a fortune staying at those. My ds did love when we stayed at the paradise pier, but I couldn't afford it for the whole holiday so we moved there for the last 4 days (which cost the same as staying the first 10 days at the ramada inn maingate!) He agreed that next time we'd spend the money on days out rather than the hotel though.
If your budget is tight you can do it quite cheaply by shopping around and booking things independantly. But if you're only going to go the once you might want to splash out a bit more.
If budget is your main priority, then make a list of everything you need and budget each item. Insurance, flights, hotel/villa, park tickets, car hire, spending money, food over and above what you would spend at home, travel to and from airports etc. Then look at what the packages offered are and go from there.