item creation staff: spell with variable material component cost

[Cuttler]

Ok here's a tricky question:

Using the create staff feat, I would like to include in the staff the spell: stoneskin communal (from ultimate combat)

Assuming Stoneskin is the highest level spell of the staff, how would you compute the value of adding that spell to the staff knowing that stoneskin communal has a material component cost of 100 gp per creature affected?

According to CRB, the spell itself should cost (400Xspell level X CL with minimum CL 8): in this case 400 X 5 X 9 = 18,000 gp
however, since the spell also has material component, we must include 50 times the material cost.

How do you compute that value if it is variable??

thanks

[Robert A Matthews]

The highest number of creatures you can touch during casting is 6 so if you have 50x6x100 = 30,000gp worth of materials, then you'll have your bases covered.

[James Risner]

Yes you need to pay the material costs.

Look at the UE for Wand of Restoration. Notice the cheap one 26,000 gp can't be used to restore negative levels with the 1,000 gp material component but the 81,000 gp one can be.

So yea go with max number you can target and multiply by 50 and 100 gp.