It's been suggested before, and found to not be balancing.
Just remove the quick buildings and it'll be just right.
Well, we're both entitled to our own opinions

.....
but I assure you I tested it very extensively.
The problem with financial is NOT cheap buildings (by the time of banks and grocers, they become largely irrelevent). The problem is that everything in CIV has an accumulative effect, and almost from the very start of the game, a financial civ of the same size as a non-financial one, can make often 25-30% more commerce. This leads to techs first, which lead to faster development (freebies also), and so on and so on. They can also afford more cities earlier, to support more units, to fight more wars etc, etc.
Its this snowball effect that gives financial civs their overpowering effect, and it all has its roots from the very start. The 3

on 2 or more plots is the sole cause of this. Take that away, and slightly ****** their development, (as I said earlier, stopping all coastal and lake tiles from becoming better or equal to others hamlets), and you accomplish this. By pegging back their runaway science monster ability, until later, is imo the only way to do it.
As someone else said somewhere on these forums, "playing a financial civ is like playing a lvl lower than you normally do". I have to concur.