IIRC the Edinburgh plan wasn't really a congestion charge, simply a charge for crossing a cordon (the ring road?). It might have reduced traffic entering the city from outside, but it didn't offer a mechanism to deter road usage entirely within the boundary.
The tram might have cost a bit less if they'd settled for COTS vehicles too, rather than insisting that Edinburgh deserved tramcars custom built to be bigger and heavier than anywhere else on the planet, which had the secondary effect of ensuring there was no resale market for the trams made surplus by the restricted build. Piss poor contract and project management didn't help much either.