Parents visitor visa: How to answer this question please?

[Somerset West]

I am inviting my parents to visit me in the UK for around 20 days. I will be paying for the entire cost of their trip (which I have estimated to be around 2000 GBP).

The application form has this question for my parents:

'How much money are you planning to spend on your visit to the UK?'

Should they say 'zero' or should they mention '2000 GBP'. Technically, they are paying nothing and I will be covering the entire cost.

I have checked previous threads and there is no clarity.

I will be grateful for your reply.

Thank you so much.

[Somerset West]

Please could someone help me with this question. I will be grateful. Thanks.