I got this info from the City of Boston website:
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The Rates of Fare are as follows:
Meter Rates:
* $1.75 for the first 1/8th mile or less
* $0.30 for each additional 1/8th of a mile
Waiting Time:
* $24.00 per each hour
Flat Rate:
* Dedham and Lexington are no longer flat rate communities and shall be charged metered rates
* $2.60 per mile (as quoted in the Official Flat Rate Guide)
MassPort and Mass Turnpike Tunnel Fees:
* All Trips to Logan Airport: $ 2.25
* All Trips from Logan Airport: $ 6.00
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I'm fairly certain that this rate structure applies to Cambridge and Somerville as well.