This is an age old question and their is no single answer. It varies by your choices and your budget.
1-Getting there- Flying or driving (if flying are you renting a vehicle?)
2-Accomodations-Staying on site, off-site, with family/freinds, in a time share (remember different levels of Disney resorts-Deluxe, Moderate and Value)
3-Park tickets- Hoppers or not, Water parks or not
4-Dining- If staying on site will you be there during free dining? Do you want to utilize table or quick service? Bring food in with you? (remember no glass)
5-Souveniers- You MUST budget for these
6-Your party- size and age matters for all these costs
It could cost $2000 or it could cost $20,000. It depends on your choices and your budget. No matter what you spend, remember to ENJOY!!