According to Scottish Government documents, the costs rose from £6.9m last September when public hearings began.
Former Lord Advocate Lord Hardie is presiding over the inquiry, which is nearing the end of its proceedings.
Scottish Conservative MSP for Lothian Miles Briggs has previously accused the probe of “spiralling out of control”.
It was launched in 2014 by then First Minister Alex Salmond. The inquiry team’s remit is to look into why the tram project ran hundred of billions of pounds over budget, endured delays and was reduced in scope.
A figure of £375m was given for the projected cost of the project, which was due to finish in 2011.
However, the tram line only opened in 2014 with nine stops, reduced from a planned 15 stops, at £776m. The final cost is predicted to be £1 billion.