Yes excursions CAN often be done cheaper by doing it yourself.
However note that if you do a trip as an official excursion, and the coach is late back, the cruise ship WILL wait for you.
However if you do any sort of DIY trip and you are late back the cruise WONT wait for you so you will be stranded at the port, perhaps without your passport, money etc!.
Daily tickets for US theme parks are NOT cheap (they want you to buy the multi day tickets - 5 days, 7 days etc) so make the single day VERY expensive.
It says here the 1 park 1 day ticket is 115 dollars (£85)
https://www.universalorlando.com/web/en/us/tickets-packages/park-tickets/promo/index.html
These parks are huge so you can see plenty in a day. Note however the car parks are huge so it can take a while to get to and from the coach park into the actual theme park so my guess is you may have more like 5 or 6 hours in the park.
Note often cruise ships don't dock till say 8am and leave at say 5pm (unless they are in port for 2 days) so your shore trip has to start and finish within those times.
I have done a couple of cruises (in Europe) and been to the USA a number of times on business and holiday if you need more help.