In fact, the point is not so much fuel saving. By reducing T/O power, you will not only extend your roll, but also reduce the climb rate until selection of climb power. So you will remain at lower altitudes for a little longer, where the engines tend to use more fuel than they have to.
The thing that makes a reduced power T/O save money is the reduced wear on the engine. By reducing power, you will also reduce RPM and most importantly the exhaus gas temperature that the turbine has to digest. Consequent use of reduced power thus has the ability to extend engine lifetime and therefore saves rather on maintenance costs than on fuel.
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