Merging West Midlands Police with neighbouring forces could add £30 to the average council tax bill, new figures have revealed.
The controversial proposals would increase bills for households in the area covered by West Midlands Police.
However, it could mean council tax would fall in the areas covered by neighbouring forces including Staffordshire Police, West Mercia Police and Warwickshire Police.
Opponents of the merger proposals said the added cost was another reason to scrap the plans.
MP Caroline Spelman said (Con Meriden) said: "Not only would we have a more distant and less accountable police force, but we would have to pay more for it too."
The Home Office is considering creating a "strategic regional" police force by merging all four constabularies.
If the proposal went ahead, the police precept, which is added to council tax bills to help pay for local policing, would be the same across the region.
But at the the moment, it is much lower in the area covered by West Midland Police, such as Birmingham, Solihull and the Black Country, than elsewhere.
West Midlands residents in band D homes pay a precept of £83.68 while in Staffordshire, for example, the precept is much higher, at £145.28.
According to figures produced by West Mercia Police Authority, a merged force would charge a precept of £115.95, if spending levels remain the same. This would mean bills in Birmingham and the surrounding area increasing by £32.27.
Even if the figures change, the trend would be for bills in the area covered by West Midlands Police to increase, while those in the other police areas fell.
Charles Clarke, the Home Secretary, has warned mergers are necessary because smaller forces do not have the resources to deal with organised crime and terrorism.
He is also considering merging West Midlands Police with Warwickshire Police, and Staffordshire Police with West Mercia Police.
Police authorities have until December 23 to present their final submissions.