The recent revelation in Avengers are funded in the MCU. Marvel Studios started answering questions about what led to the Maximoff Anomaly, with an in retrospect outing that tackled all of the trauma Scarlet Witch has endured throughout her life leading to her breakdown. The death of Vision and her subsequent realization that he's truly gone tipped her over the edge, with her grief resulting in her rewriting reality within the hex so she can live the life she's always wanted with the synthezoid.
WandaVision didn't confirm when exactly Vision purchased the property in Avengers: Infinity War as they secretly dated. At this point, Scarlet Witch was dubbed a vigilante and was with Captain America and the Secret Avengers laying low. As revealed in Infinity War, however, she and Vision would regularly meet up — something that both Steve Rogers and Tony Stark knew.
Still, this doesn't explain how Vision had the money to buy the land. Granted that Westview isn't exactly an upscale real estate haven, it would still cost him a bit of money to acquire it. Given this, it's curious how he and the rest of Avengers: Age of Ultron when he told Maria Hill, "I just pay for everything, design everything, and make everyone look cooler." Both Avengers headquarters were also originally Stark properties: the Stark Tower in New York and the old Stark Industries warehouse in the upstate.
Considering this, it's safe to assume that Stark also set-up accessible funds for the Avengers, particularly the ones who have devoted themselves to the superhero cause. While former military men like Captain America and Sam Wilson may still get compensation from the government, Hawkeye, Black Widow, Wanda, and even Bruce Banner may have been financially ed by Stark. Thor is well taken cared of when he came to Earth, but since his solo adventures were mostly off-world, he didn't demand a lot of funding. As for Vision, considering that Stark has had a long working relationship with him starting as his AI assistant, JARVIS, he may even be a bit more generous with him, explaining why the synthezoid had the money to buy the Westview property.
In any case, even if there have been other streams of income for some of the Avengers, Stark would still be their primary financier which in hindsight, was probably for the best. This way, they're not beholden to any entity and were able to operate with autonomy, until the events of Civil War. Now that Avengers: Endgame, but if things are unclear, it makes it more difficult for his contemporaries to move forward in the universe and assert their agency with the Sokovia Accords back in effect.
