We recently went on vacation. To Disney World. And Universal Studios Florida. $$$$$ and $$$$$ and $$$$$. I'm still calculating how much money we spent.
There were four of us, and we got a discount (listened to someone's spiel about time shares) on tickets for two of the 4 major parks at Disney World - $500 ($659 without discount). That's two each day with no park hopping ($879 for park hopping between the big four parks). Parking each day you go is currently $14 so even if you spring for the extra park hopping tickets you still have to pay for parking each day. Food for four runs about $35 a meal, times two for lunch and dinner if you stay at the park all day. You can bring in your own food, but no coolers so bring non-perishables. We brought in water bottles and transferred the ice after our lunch. We also tried to take advantage of the Fast Passes when we could. Fast Passes don't mean extra money at Disney...
But they do at Universal. To get their quicker line deals you have to pay more and parking costs $15. However, the park hopping is better with only two parks ($20 more per person). Two day park hopping tickets for 4 (3 adults and 1 child) were $569. The meals and food were about the same as Disney, however they did have a special on the Harry Potter Butterbeer mugs - buy the mug for about $10 and get all soft drinks with that mug for only $0.85 instead of $2.19. Look for any of those deals. Just as a note the Universal in Florida has something called City Walk (read Night Life) and many stay after the parks close for this.
Going to amusement parks now is sooo much more than when we were younger. Find any way to make your costs cheaper. Go off season, get package deals if at all possible, take food into the park (which is much easier when the park is near your home), wear decent shoes (walking every day for 4 days is killer on an old bod'), wear a hat in hot weather (one that can easily attach to a belt loop so it doesn't fly off on the rides), bring in your cash and card (ATMs in the park have extra charges), and don't forget sunblock. You don't want to have to buy anything inside the park if you can help it.
I didn't include souvenirs, lodging, food outside the parks, car rental, gas, and airline tickets (in our case). I'm sure I'm forgetting something. What I'm trying to say is think carefully and plan well. Then enjoy :)