New figures revealed that Cardiff council received £18m less cash for affordable housing and community facilities than it asked for from developers in the past five years.
As outlined in reports, some developers paid nothing, as requests for £21.7m across 25 schemes resulted in deals to pay £3.75m.
The figures were released following a freedom of information request from the Local Democracy Reporting Service.
The council says while it asks for the maximum it can, the district valuer decides what the developer can afford.
Local government expert Dr Marlene Davies said: “Developers are out to make money – end of story.”
Funding for affordable housing was often the element cut or scrapped from the deal, according to the figures.
Projects such as the city’s new bus station and flats in former industrial areas of Butetown are among those where the agreed sum – known as a Section 106 payment – was a fraction of the initial request.
A Cardiff council spokesman said the process was a “negotiation” with the developer.
“Many people will see a proposed multimillion development and assume there is excess profit to be made,” the spokesman said.
“What isn’t always known is how much it will cost the developer to buy the land, the cost of the building materials and the cost of labour to build the development, often many years after planning permission has been sought.
“These costs then need to be offset from the actual income they may receive when they sell the homes on the open market.”
Dr Davies, from the University of South Wales, said councils knew it was “a game that they’re playing” and probably did not expect to get all the benefits they asked for.
“Local authorities have a duty of care to the public and they have to try and get as much as possible,” she said.
“Quite often the people representing the developers are probably a bit savvier.”