Almost €3m has already been spent on engineers and consultants for the BusConnects plan in Dublin.
New figures show a US-based consultancy firm, that won the tender in 2016, has been paid more than €600,000 for work up to August of this year.
Another 11 consultancies have shared €2.2m to date for their work.
There has been widespread confusion about the Bus Connects proposal, which would transform the look, layout and frequency of routes across the city.
Sinn Fein spokesperson on Transport Imelda Munster is concerned about the outsourcing of work.
"When I had first seen the figures, it was quite shocking to see such an exorbitant cost to the State," she said.
"It certainly puts into question the setting up and funding of State bodies such as the NTA and TII, if they're void of any expertise."