Merging councils in Wales could cost £200m, twice the estimate of the report demanding change, said the Welsh Local Government Association.
The Williams Commission proposes merging local authorities to reduce numbers from 22 to between 10 and 12.
WLGA chief executive Steve Thomas said there was no evidence the last council reorganisation created major savings.
But the commission estimates this latest shake-up could save £60m to £80m per year after an upfront £100m cost.
Commission chair Sir Paul Williams has said council mergers using existing boundaries should be under discussion by Easter.
It would be the first council reorganisation in Wales in nearly two decades.
Mr Thomas said: "How are we going to pay for this? There are quite a lot of upfront costs. The work that we've done suggests something like £200m.
"Do we want to spend that £200m on reorganisation or do we want to spend it on services?"
In 1996, the current system of 22 unitary authorities was created out of eight counties and 37 districts which were in turn formed in 1974.
Mr Thomas said his experience of the 1996 boundaries shake-up was that there had been no report detailing the savings it led to.
"I was involved in the 1990s reorganisation and I can't find a definitive figure from that reorganisation to suggest that there were huge savings," he said.
"There may well have been but nobody's done a definitive piece of work on that."
The WLGA has said research it commissioned by accountants Deloitte suggested reorganising councils in Wales could also cut 15,000 jobs.
The Williams Commission estimates that much of the £100m cost of reorganisation would be in redundancy payouts.
The current single unitary authorities were created with the aim of being more efficient by cutting out a layer of bureaucracy and making it simpler for people to understand who was providing their public services.
Proponents of change argue that many of the 22 councils are too small to deal with areas such as education and social services, although it is not just the smaller authorities that have been criticised for the quality of their services in these two areas.
Electoral Reform Society Cymru said it had warned the Welsh government not to rush into making changes because local democracy could suffer.
Director Steve Brooks said: "Larger councils could make voters feel more disconnected and less able to influence decisions which affect their everyday lives.
"Evidence from Europe suggests that this can undermine the public's confidence in local government and its ability to respond to local needs.
"Without voting reform, there's an added danger that these larger councils will become one-party states, which will only further compound councils' governance problems."