As a starting point, forget about materials, concentrate on the time/labor. Time out how long it takes you to do a typical 'standard' bowl, say 6"x3", relative to one of your pens. If it takes twice as long, it should be priced (at least) twice as high.
That gives you a base-point for pricing: price the rest of the bowls proportional to their diameter*height. Then you can apply an adjustment factor for things like exotic/purchased blanks or extra steps like segmented feature rings etc.
Finally, do some sanity checks: "Is this bowl priced correctly relative to that bowl? Would I laugh out loud if I saw that pricetag on a bowl in a store?", things like that.
And if you get to the end of the process and find yourself raising the prices on your pens, that's ok too.
Where are we going? And what am I doing in this handbasket?