We are local and have Inspire keys. We've probably gone 25+ times, so the math would say we've saved thousands of dollars. The one day tickets are so expensive now ($200+ with park hopper) that we probably would never go without MKs, or if we did it'd be like a 3 day, stay at the DLH staycation thing.
That being said, we end up spending TONs of money when we go, so its certainly not as simple as "go ~10 times and you'll net savings." We get G+ probably 1 out of every 3 times we go. Food, drinks. We're not big on merch, but occasionally my wife wants to get something... we spend $50-150 every time we go as a family of 4, unless we don't eat. We have started bringing our own food but that takes away a lot of the spontaneity. (as an aside, I'd pay $200 for a full year G+ add-on and just never have to think about buying it or not again!)
Also, if MKs didn't exist, we'd just do other stuff like go to the beach or a public park, or the zoo, and probably be just as happy. So it's impossible to say. For us, we love being able to decide night before or morning of to go (reservations generally aren't an issue if you keep checking) and we would just not go nearly as much if we were paying for tix each time. But it's not a money decision for us at all.