How much rent insurance costs depends on which of two products is meant, and they are priced completely differently. One is a section of a policy you already have. The other is a separate purchase with its own premium. Asking the price without saying which produces an answer that is wrong either way.
Loss of rent: usually no separate price
It is a section of the buildings policy, normally included as standard and expressed as a percentage of the buildings sum insured. There is no separate premium line for it in most quotes. Extending the period from twelve to twenty four months does carry a cost, and it is usually small.
Rent guarantee: a real separate premium
Quoted either as a fixed annual amount per tenancy or as a percentage of the annual rent, varying with the cover period, the excess period and whether legal expenses are included. This is what people usually mean when they ask what rent insurance costs.
Comparing them is meaningless
They answer different risks. One responds to the building being unusable, the other to a tenant not paying. A landlord choosing between them on price has misunderstood the products; most landlords have the first already and are deciding whether to add the second.
Questions people ask about how much is rent insurance
Is loss of rent charged separately?
Usually not. It is part of the buildings section and included as standard, expressed as a percentage of the sum insured.
How is rent guarantee priced?
Either as a fixed annual amount per tenancy or as a percentage of the annual rent, varying with the period, the excess and the legal expenses.
Which one do I already have?
Almost certainly loss of rent, as part of your buildings cover. Rent guarantee has to be bought separately or as a named add on.