Most inexpensive - it's not close - offsite.
However, you may or may not enjoy being offsite, so your next most inexpensive path will be renting DVC.
To be inexpensive, you'll want to have 2 of 3 meals a day in your room, so a kitchen will be essential. You'll want to avoid Disney character and AYCTE meals b/c with 5 Disney adults, you're looking at $250+ for those meals. You'll want to keep onsite dining to CS and cheap 1 TS options...or to be even cheaper, eat at restaurants offsite. You'll want to bring snacks and drinks to the parks...
As for tickets, there's not much to keep it cheap. The best plan would probably be to wait to see if Disney keeps offering the 4 day, 1 day in each park special...but if not, you'll want to weigh how many days you want to tour...and look to see if other sellers can meet your needs a little cheaper.
You're already driving, so that's a cheap way to get there
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Other ways to be as inexpensive as possible - take your own pics, ignore the souvenirs, and avoid the upsells
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