$15 per color, one time cost
Rest of pricing is the same as normal.
+ any custom ink color mixes needed at $15 or 25 per mix, one time cost.
We should probably be rolling in a couple color changes and a base amount of sampling time because every sim pro and process job requires at least one or two ink pulls and adjustments.
Another thing we should be considering is the mesh costs are much higher to run these jobs. 330/30 screens like to break sooner than others and we use them almost exclusively for sim/process top colors.
Toss in that some of our sim pro jobs run WB/DC and there's a lot of wiping out while making adjustments.
So I'd say Pierre's price per color is a better figure overall now that you made me think about it.
If you think your clients would respond better to set cost and wanted to flat rate it I would maybe do something like 7 or 8 colors x cost per color + all that stuff above and anything else you can think of rolled in. You'd have surplus pay on the lower color count jobs to make up for squeaking by on the larger color count ones.