Using 260 amp consumables, 500 pierces per set, and approx 60 seconds per cut. If you only consider consumables, gasses, and electricity your approximate costs per foot of cut for mild steel would be
1/4" = $0.008
1/2" = $0.013
3/4" = $0.021
1" = $0.03
As you can see these are very low which is why the HPR series of industrial high definition plasma systems from Hypertherm are the choice of most plate cutting houses looking for the lowest possible operational costs.
The only way to come up with a cost per hour is to take into consideration the "human factor" i.e. material handling etc.
The real expense is operator labour. Increase "arc-on" % per shift and your operational costs drop like a stone. Typically this is achieved by loading and off-loading your machine more efficiently and miminizing the time require changing consumables.