Figure of merit of Operational amplifier
First, you define what your Figure of Merit (FOM) means.

I've sometimes (but not often) done this when comparing op-amps for a new application. Once I needed to make a low distortion video driver into 50 ohms, and had the choice of one beefy opamp, or several smaller ones in parallel. I defined an FOM as the output current / supply current for a given amplitude and distortion. I eventually used 3 in parallel.

Usually, FOM is only a good measure when you expect two parameters to scale with each other, and you have some freedom about where to operate (like how many opamps in parallel). For instance, if you were building synthesiser, you could use a 10MHz reference, or a 20MHz reference, with different multiplication ratios. Low noise oscillators have an FOM of phase noise / operating frequency, so it makes sense to pick the best ratio, and design the multiplication chain starting from there.

However, most opamp specifications do not scale, a ratio is not appropriate. You might need a particular bandwidth, or input impedance, or input offset. There the choice comes down to listing all the opamps that meet or exceed all of your specifications, and choosing the cheapest supplier that you trust. It does not make sense to define an FOM as (say) bandwidth per dollar, if your application requires a particular bandwidth. 

An FOM of bandwidth per dollar could be useful in the marketing department as a way of showing you make better opamps than the competition.