Estimates for the cost of electing 36 more politicians to the Senedd – likely to be more than £12m a year – are to be published for the first time later.
Labour ministers will publish their plans to expand the Welsh Parliament, including more politicians and a new voting system, on Monday morning.
The changes, if ready in time, will take effect at the 2026 election.
The plans have the support of Plaid Cymru but are opposed by the Welsh Conservatives, largely on cost grounds.
Up until now the Welsh government has not put a price tag on the reforms, which were drawn up between First Minister Mark Drakeford and Plaid Cymru’s former leader Adam Price last year.