Calculating the cost of the trip is step one:
Tickets will be your biggest expense. Arriving in June and wanting 5-days in Disney, you'll pay about $400 (that's basic tickets, no park hopping). Universal tickets don't really come in a 2-day option, and a 3-day will cost you about $270.
Options to lower this cost:
- Stick to Disney alone; you could add more days to Disney less expensively than you could do both parks.
- Don't do the parks every day -- realistically, 5-day tickets are "just right" for a one-week stay; for example, if you go to the parks on your first and last day (your travel days), you're spending a ticket for a partial day (especially once you add in transportation around the . Instead, do something at a resort on those first /last days ... you can visit Disney Springs without a ticket, you can watch fireworks without a ticket, you can visit other resorts without a ticket, etc.
- Plan a mid-week no-park day ... sleep in, visit a water park (less expensive than a "real park"), then have a nice dinner.
- Plan your days; that is, make the most of every day you "spend a ticket" ... okay, this isn't a money-saving tip, but it's a make-the-most-of-what-you-spend tip.
Hotel ... someone else asked about a credit card ... will Disney take reservations from 18-year olds? I really don't know. Staying on property is probably smart because it'll include your transportation, and it allows you to split up into smaller groups. Why are you against the least expensive hotels on property? Your intended ticket purchase makes me think you intend to go-go-go all the time ... why, then, not choose the All-Stars, if it's just going to be a place to crash and shower?
This isn't a money-saving tip, but it's important: If you're looking to get an inexpensive place to stay, you need to make your reservations EARLY. Making reservations this summer isn't too soon. LOTS of people want those least expensive rooms.
Food at Disney is more expensive than you expect. You can check out menus online; don't ignore taxes and tips. If you add 15-20% to what you see on the online menus, you'll be just about right. You mentioned meal sharing; note that sometimes sit-down places add a dollar or two for the extra plate -- yeah, stingy. Kids's meals are a budget option, and they usually include a drink. Bringing your own snacks from home is probably your cheapest option. You can order groceries ahead of time /have them delivered to your room. On park days, you can ask for free ice water at any fast food restaurant.
On that subject, take everything you think you'll need: Aspirin, sunscreen, etc. These things are so cheap at home, but extremely expensive to buy on Disney property.
You intend to fly? That's expensive too. Don't neglect ground transportation; that is, how will you get from the airport to Disney's property? Consider, too, transportation between Disney and Universal.
At a glance ... I'd estimate you'll need about $1500/person + the plane flight. This'd give you tickets for both Disney and Universal, 1/4 of an All-Stars room, and food -- and an Uber or two, but no souvenirs or fancy meals.