What an absurd claim and article? As someone in the UK I estimate it was would cost probably a less than a couple of hundred for the paint, supplies, and ladders. Probably less if they bought the paint and did it themselves.
The problem is that they feel obligated to protect it from vandals and thieves.
As mentioned elsewhere, the whole thing may be essentially marketing to find a buyer.
We had a relatively recent Banksy not far from where I live (Bristol, UK so we do have quite a few around the place): https://www.bristol247.com/news-and-features/news/has-a-new-... . The council quickly put a protective plastic covering over it.
Otherwise, the house owners could just attempt to sell it and specify that the buyer has to cover the costs of removal and repair.
Here's an example in Totterdown (also Bristol) that appeared whilst the house was about to be sold: https://www.theguardian.com/artanddesign/2020/dec/12/owners-...
But all I was asking about -- whether owners were "forced" to keep it? Because that is vandalism. Now you like that on your property after the deed, or not, if up to you. Not upto the vandal/artist and should not be up to the city council.
There's even an example of an old codger painting over a Banksy caught on camera here: https://www.bbc.co.uk/news/entertainment-arts-59236187
The title is.. well clickbait really.