I think this all comes down to how do you want "to do" WDW? There are so many different ways. There are some things that will be relatively fixed "hard" costs. For example, if you have to fly down then while you can look for bargains it's a cost you have to incur no matter what. Park tickets are what they are, not much you can do with that.
Then there are the personal choices which will dictate so much about how expensive or inexpensive your trip is and/or how much work/planning you do. Onsite or off? Deluxe, moderate or value, and how important are those things to you? Dining plan or not? QS or TS? Bring your own food in? Park hopper or not? Water park or not? If yes, rent an umbrella/cabana or not? Special ticketed events or not? Plan for every minute of every day, or just wing it? I've read of some people literally planning how long it takes to walk from attraction to attraction, and on the other end people who walk in with zero plans whatsoever. Where do you fall in that and how important is it to you? So many variables here.
For us, we can drive down so no flight costs. We like/need to do WDW on the cheap, so we could do a 9 day trip to WDW for our family of 5 for under $5000. That would include park tickets (hoppers with water parks), lodging and food. We'd spend less than an hour planning the entire trip. But that's just based on our own personal preferences. For someone else it's a very different (and not right or wrong) story.