Taxing the poker machine profits of clubs at a lower rate than pubs in NSW will have cost more than $13 billion over 20 years – about the same as what will be spent to build the Sydney Metro rail line.

Of that, decisions by the O'Farrell and Baird Coalition governments to further reduce pokie taxes for NSW registered clubs since 2011 are forecast to have cost more than $500 million in foregone revenue.

The figures reflect the cost of a policy upheld by successive state governments to tax poker machine profits of registered clubs at a lower rate than those in pubs.

The policy has been in place since April 1997, when pokies were first allowed into NSW hotels and were taxed at a higher rate by the Carr Labor government.