I am currently in the early stages of planning a trip to Universal. (We have been there many times but it's been a while.) I did see that they're currently offering a ticket promo: Buy 2 days, get 2 days free. Maybe spending more time at Universal and less at Disney could make this worthwhile for you.
Undercover Tourist is offering it too and cheaper than buying it directly from Universal. If any of you belong to AAA, you should inquire about a AAA discount at Uni for hotel rooms. Not sure if they still do but they used to offer that.
I agree with others about checking on flights now and check various dates. It can vary a lot. Generally speaking, Tues & Wed are the cheapest days to fly. Balance that with hotel prices being lower Sunday - Thursday. Friday & Sat nights at hotels are sometimes much higher. If you keep checking often enough, you will immediately recognize a good deal when you see it and be ready to pounce on it. A few hours later it might be gone.
A pp mentioned reading thoroughly about the airline you book. Know how much luggage fees, seat selection fees if desired, change fees if necessary. You can factor all of this into the total price of flying.
I agree with the pp who mentioned packing as much food as you can. A box of cereal, instant oatmeal (run hot water through the coffee maker) and some PB & J will help cut down on meal costs big time. (Seriously, I have packed a loaf of bread in a carry-on!)
There are many ways to save and splurge at both parks. Some folks prefer a cheap hotel and splurge on character meals. Others value a higher resort level and are happy to pack some food in order to save on that end. There's no "right" or "wrong" way to do it, just a matter of taste and priorities, but if you're not all in agreement then it will be hard to please both sides. It's good to have a group discussion of this in the early stages.
First thing though is to check if an 18 yr old can check into a Universal hotel. You need to know this before you make any more plans. Three of their hotels include unlimited Express Line privileges.
(Royal Pacific, Hard Rock & Portofino Bay). That is heavenly if you can swing it even just one night. That gives you 2 full days of Exp Line: the day you check in and the day you check out. Skipping the lines drastically cuts down on the amount of time you need there overall.