IJR
The ACI code does specify a maximum amount of reinforcing for flexural sections. The reason is to ensure that the beam or slab will fail in a ductile mode.
For a given sized cross-section, a small amount of steel will provide a section where the steel will yield BEFORE the concrete on the compression side will crush.
As you increase the amount of rebar, at some point the rebar will be sized to yield at the same time the concrete crushes. This the called the balanced condition.
If you increase the rebar more than this, then theoretically, the concrete will crush first, prior to steel yield. This is a very abrupt, catastrophic failure of the beam which is dangerous in that there is no prior warning of a problem.
The code limits the rebar to 75% of the balanced amount. This is to ensure that, if the beam is overloaded, the steel will yield, producing a gradual, cracked section...warning the occupants of a problem.
For your main question....what is economical... that depends on the relative cost of rebar vs. concrete in your area. Also, deflection limitations many times give you a minimum slab depth that is recommended. The ACI code, for instance, recommends that one way slabs have a span to depth ratio no larger than 20. This gives a good starting depth of slab that can be designed for strength.
The Means cost estimating guide can also be used to compare the relative costs of thinner slab/lots of rebar vs. thicker slab/minimal rebar.
