So: I ticked 'yes', and here's how I imagine it happened:
1. In S5, Willow and Tara were living in college accommodation. Presumably they were paying rent for that, although it was never mentioned - nor were we ever told how they afforded it. Loans, scholarships, money from Willow's parents, off-camera jobs that were never, ever mentioned, or even that spell to "excrete gold coins" that Riley and Anya talked about...
2. Buffy dies. Dawn is left alone, and she's 14; too young to look after herself. I imagine the idea of sending her to her father was raised briefly then dropped again when Dawn violently rejected the idea.
3. Sending her to live with Giles, or Xander and Anya, was probably unwelcome in both directions. She couldn't stay in Willow and Tara's dorm room; but they could come and stay with her to look after her. At first I think this would be seen as temporary, while they decided what to do.
4. The university year ends, and Willow and Tara have to decide whether to rent their room for another year. At this point, my belief - neither supported nor contradicted by canon - is that they decided to move into the house on Revello Drive instead and pay whatever rent money plus household expenses they'd been paying on the dorm room, for the house instead. In effect, and legally, they were Dawn's lodgers, renting their room from her.
5. The rent money they paid for the room wasn't enough to cover the mortgage. At least temporarily, Joyce's savings could supplement that; but medical bills ate away at the capital. I'm assuming that some of these would only be sent after Joyce's death? Or maybe she'd never got around to paying them before she died, and the creditors were starting to send nasty letters? (Incidentally, I assume the money was technically in Buffy's name, and since they didn't tell the authorities she was dead, Willow had to magically forge her signature...)
6. We know from Season 8 that Willow and Tara at least considered the option of moving away from Sunnydale, raising Dawn as a foster daughter, and presumably therefore selling the house and buying something smaller and more manageable. But instead, Willow chose to raise Buffy from the dead.
7. Once she'd decided to resurrect Buffy, I think everything financial was simply put on hold. There was enough money left in the bank to tide them over for a few more months, and psychologically they couldn't sell Buffy's house if there was a chance she'd come back to it. They just assumed something would come up and everything would be all right once Buffy came back. (Yes, that was shortsighted and wrong of them, though also understandable.)
8. When Tara left Willow after 'Tabula Rasa', she presumably went back to college accommodation (in 'Villains' Willow goes to Stevenson Hall to cast her locator spell). That means the amount of money coming into the house would be halved - just Willow's contribution - but at about the same time Buffy started working herself, so it balanced out.
9. In Season 7, I'm not sure what arrangements Willow made. Given how remorseful and controlled she was, I can't imagine her not volunteering to help with the expenses again, though perhaps on a less formal "however I can help, I will" basis rather than an actual rent payment.
10. In early S6 Willow probably came up with all sorts of schemes to use magic to cover the household expenses. Tara doubtless vetoed them.
In summary: it seems out of character to me that Willow and Tara wouldn't make any financial contribution to the cost of the house - but barring magic, there's no way they could afford to pay for it all themselves. I wouldn't completely rule out the idea that they simply didn't think of it, and that Buffy could never bring herself to ask them for money - but it seems the less likely scenario.