In many cases one would not need an "extra" amplifier after the divider. A stacked reference may also need an amplifier to buffer the output, but this one is way less demanding: the stacked reference is low impedance (like a few ohms, depending on the refs used). A higher voltage divider tends to be relatively high impedance (e.g. 20 KOhms) even at the output, especially if only something like 10:1.
There is another use for a stacked reference: one can compare/measure the individual reference and this way do something like a 10 V to 100 V step. Affordable dividers are usually not that long time stable. It takes something like a Hamon type with periodic adjustment / check to get a long time stable / accurate ratio. Even there the accuracy may be limited by self heating effects and leakage currents.
If a suitable meter is available it could be worth to have all the switches needed to choose between series and individual (maybe also averaged) use. For averaging it depends on the references how much of an effort is needed - with low impedance references one could directly use resistors for averaging. It also depends on the effort for the current supply.
Stacking reference is the accepted first principles way (by definition of voltage) to add voltages. A divider would need calibration.
A cost comparison is not that easy, as many reference in series / parallel are also lower noise and could cancel out some drift peaks. So if would be more like comparing something like a stack of 14 LM399 based references to an LTZ1000 based with step up amplifier. The cost could be comparable, but the performance would be different: Chances are the stack is better at 100 V, while an LTZ1000 is better than the average over 14 LM399 at 7 V. So it depends on the use.
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