There are so many factors at play, there's no black & white answer (IMO). We do both off site and on site -- different variables at play for all trips. I try to keep costs similar even when the places we stay, rent car or don't rent car, etc. are all different.
We also fly, so I'm with you there on airfare costs. We have a Southwest Rapid Rewards Visa that we were able to use to pay for 5 round trip flights to MCO 2 weeks ago. So that's a big help. But if we can't get good airfare deals, it makes it tough.
Typically if I'm staying off site, I'm looking for something more 'deluxe' than I can afford (or care to pay for) on site, like a 2+ bedroom condo or large suite. We're a family of 5, so we are out as far as most basic double occupancy standard rooms go... so if we're off site, I'm usually keeping an eye on something like Lake Buena Vista Resort Village, Wyndham Bonnet Creek, or Embassy Suites. I think for a standard double occupancy room, 4 walls and 2 beds and pool access, you can for sure find cheaper deals, and you can also take advantage of things like the blind deals on Hotwire (I can't). But then yes, you have to then factor in a rental car on top of the room rate. Being 5, and all of our kids are in boosters, we have to rent a minivan or SUV to ensure all the seats fit which seem to run between $350 - $400 for a week.
If we're off site, I like a kitchen or even just a kitchenette so I can do a couple quick meals there to save some dough. I don't want to cook the entire week though. I already do that when we go camping or when we rent a house in Outer Banks or Ocean City or wherever, so I don't have a strong desire to do it on a WDW trip. But even so, I like to do at least a few meals in the condo. If I'm on site, in a regular room, I get an
Amazon Prime Now order and pack some park snacks and do easy breakfasts in the room. I also only do a small handful of ADRs and stick with quick service or snacks for other stuff. We also split a lot of things. My kids aren't big eaters so this makes it easier. When they are teenagers we will probably have to modify that. I also enjoy laundry access, I don't like to pack 7 tons of clothing for 5 people so I pack half a week's worth and do a quick load... I also like coming home with clean clothes to just put away. So condos with washer/dryer are cool. I thought this would be an issue at our recent on-site stay, but it wasn't... the laundry room was close and you don't have to stay there -- they even have it set up to send you a text message when your load is done. So both worked out.
Again... just so many factors at play. Sometimes one works better than the other. Some things are more important for some people and not important at all for others. The trip we just got back from we were on site. I paid $147/night for a room that sleeps 5 at Caribbean Beach Resort and used the bus transportation. The buses aren't my favorite thing ever, but we saved money on a rental that we were then able to use for other things. And there are some perks -- we could drink a bit and not have to worry about it, refilling the gas tank before returning the rental, didn't have to pay for resort parking, etc. They were pretty much always on schedule for the most part. My kids liked the "Disney bus", it was a thing for them. If my budget was totally limitless? Yes I'd rent a vehicle. Or, you could also Uber multiple times and possibly still come out less than the rental cost, depending on your length of stay and how much back-and-forth you're doing.
For me, I just do a list or a spreadsheet and do comparisons and see how the numbers add up, the pros and cons to each, etc. I enjoy both kinds of stays for different reasons. I do what works best for our enjoyment and our budget.
[ETA] If looking for just a standard, 2 queen bed room, try out places like the Holiday Inn at Disney Springs. You get some of the same benefits as on site like 60 day FP+ and EMH participation. There's also a shuttle. I just did a really brief look for the beginning of May and it was $169/night.