The chief executive and two others are on leave on full pay after the Wales Audit Office (WAO) declared their salary rises unlawful in 2013.
So far, more than £1.7m has gone towards their pay and another £752,000 has been spent on legal costs.
Officers are recommending councillors set aside another £263,000 if the case is not resolved by the end of June.
A full council meeting on Tuesday will be asked to approve the additional money, to cover further salary and legal costs.
It would take the total cost to the equivalent of £32 per household in the county borough.
Chief executive Anthony O'Sullivan, his deputy Nigel Barnett and head of legal services Daniel Perkins were suspended after the WAO declared their pay rises unlawful.
It was claimed they agreed to give themselves a 20% increase when most other council staff faced pay freezes.
A criminal case, involving charges of misconduct in public offices, was dropped in 2015.
An internal investigation into the three senior officers is now under way.
The final decision will be made by an investigating and disciplinary committee made up of Caerphilly councillors, which is being advised by a barrister.
A spokesman for Caerphilly council said: "The ongoing disciplinary investigation into three senior officers has resulted in additional costs to the authority totalling £2.5m to date.
"It is important to note that a portion of these costs were incurred during the police investigation which lasted over two years.
"During this time the council was advised that it was unable to undertake its own proceedings and therefore the timescales have extended beyond what was originally anticipated.
"All appropriate steps are being taken to progress this matter as quickly as possible, but the investigation must be conducted fairly and thoroughly in accordance with an agreed statutory process."
The cost of a long dispute over pay rises for senior officers at Caerphilly council is due to rise to nearly £2.8m.