I know that instrumentation amplifiers are difference amplifiers with buffered inputs, and I understand why one would select an instrumentation amplifier over a difference amplifier circuit. But is the only reason one would use a difference amplifier over an instrumentation amplifier cost, or are there some other good reasons/use cases much better suited to difference amps?
But is the only reason one would use a difference amplifier over an instrumentation amplifier cost?
No, one great advantage that a difference amplifier provides is substantial "beyond the rails" operation. Consider the ADI difference amplifier below: -
Look at +IN - it sets the scene for the voltages at the actual inputs to the internal op-amp. If "REF" is tied to internal ground then +IN can approach nearly double the voltage that is applied to +VS.
This cannot be achieved with an InAmp.
But, other difference amplifiers can provide even bigger limits on the terminals such as this one: -
This can operate with an external input that is about ten times greater than the supplies to the chip.