I suppose it is a good thing to have a total dollar amount in mind, but I approach it in segments. There are some costs that are pretty fixed, like park passes, and others that are moderately fixed, like room costs. The best legal discounts you can get on the passes run 5-10% off the regular price. The 10% discount is available for members of Connections, while the 5% is available at several ticket brokers operating in and around Orlando. Room costs vary by season and room discounts vary by season, resort type and discount type. The most widely known discounts are Disney Club and Annual Pass Holder discounts. These discounts are not available all the time and at times are available for one type of resort, but not the other. If you can play with your travel dates, you can work to get a great discount. This year, Disney flooded us with discounts in the form of codes. These discounts are offered when occupancy rates are so low they need to fill the resorts. Again, with a flexible schedule, you can take advantage of these discounts when available to reduce your room cost. The most varied cost are for food and souvenirs. In these areas you can go from peanut butter to filet mignon. If you snag a good room discount, you can adjust your food and souvenir costs. As for travel (airfare, driving, train), I have found that the earlier you can book, the better. It is true that better fares happen, but usually you can contact the airline and they will adjust your fare and give you credit vouchers for future travel. Depending on where you live, driving may be the least expensive option - but there are time costs (how much is your time worth) and sanity issues. Your best move is to purchase a good guide book. I recommend The Unofficial Guide to Walt Disney World by Bob Sehlinger. This book is extremely comprehensive and is very helpful in explaining how to plan your trip. Some of the information is a tad over-critical (food reviews), but I feel this book is the best for the first-time visitor to WDW. The next best tip is to keep reading these boards and ask questions. The information you find here isn't in the guide books - although it should be!
Good Luck!