Wolverhampton Council was forced to move the event – which starts tomorrow – to Aldersley Leisure Village due to the Civic halls revamp.
Now, according to an internal report seen by the Express & Star, events transferred to Aldersley are forecast to lose £45,000.
These include the darts, as well as comedy shows by Dave Gorman, Jason Manford, Sarah Millican and Joe Lycett, whose November 26 show has sold out.
The October 19 report, compiled by council officers, said: “Originally, the events were forecasted to lose £75,000, however, costs were driven down in time for the last cabinet report to the point where the events were forecasted to be cost neutral.
"Since this report, darts ticket sales have been low, and so a loss is now being forecasted.”
The Grand Slam is now in its 12th year in Wolverhampton and has always been held at the Civic Hall.
The 32-player tournament has long been seen as a money-spinner for the city, bringing in thousands of fans from all over Europe as players from both the PDC and BDO systems compete.
The council announced plans to move it along with other events earlier this year after the restoration of the Civic was delayed.
The 80-year old venue is riddled with asbestos and needs a new ceiling and rewire.
The cost of the overhaul more than doubled to £38 million and, more than two years after work started on the job, it remains in a state of disrepair.
Tory councillor Paul Singh said: “This is one of the biggest events the city hosts and the council was insistent that moving it to another venue would not have a detrimental effect.
"To now come out and say it is going to make a loss is just not good enough.”
A council spokesman said: “With the Civic Halls closed for refurbishment, councillors were determined to keep high-profile events in the city to benefit the wider economy, rather than risk losing them to other places.
“Both the council and PDC are delighted to have kept the event in the city but we anticipated it would be difficult to match the record-breaking sales of last year.
“We have worked hard with the PDC to promote this year’s bwin Grand Slam of Darts, including laying on shuttle buses for customers to and from the city centre.
“While overall sales are down, sales for some sessions are actually higher than 2017.
“In addition to the darts, we have also switched a number of high-profile November comedy events to Aldersley Leisure Village and sales for the six comedy nights match those of similar events at the Civic Halls, with Joe Lycett on November 26 a sell-out.”
PDC Chief Executive Matthew Porter added: "We're sure the bwin Grand Slam of Darts will prove as successful and popular at Aldersley Leisure Village as is was at the Wolverhampton Civic Hall.
"We've worked hard with City of Wolverhampton Council in recent months and are looking forward to an exciting nine days, while the event will receive greater worldwide exposure than ever before through our international broadcast partners.”