Car drivers currently pay £6.50 to use the M48 Severn Bridge and M4 Second Severn Crossing.
The chancellor George Osborne has said tolls for cars and vans will be cut to £5.40 in 2018, when they return to public ownership.
But Monmouth MP David Davies said there was scope for a bigger cut.
Fees are currently collected by a firm to pay for the construction and running costs due to be repaid by 2018.
Mr Davies, who also chairs the Welsh Affairs Committee at Westminster, said it was possible to maintain the bridges "on a fraction" of the cost when they revert to public ownership.
He has obtained figures from the Department for Transport (DfT) which he said showed a net revenue of £91.4m in 2014, of which £13.16m was operational expenditure, including maintenance, with £17m paid in Vat and which will be waived when they move into public ownership.
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"While we need to be careful in our interpretation of these figures, they conclusively demonstrate there is huge scope for a dramatic cut in the price of the tolls when the crossings revert to public ownership," said Mr Davies.
In a letter to Mr Davies, the DfT said a financial forecast showed a deficit of £63m would still be outstanding by 2018, revised down from £88m, and which "could be recovered through tolling".
The Welsh government has called for control of the bridges to be devolved.
Toll prices on the Severn Crossings could be cut dramatically and still cover running and maintenance costs, an MP claims.