Spell Pierce vs. Mishra's Workshop

[brokenbacon]

Hey all,
I was just wondering - take the current scenario:
Opponent controls Mishra's Workshop and a Mox (nothing else), and they play a Sphere of Resistance. If you Spell Pierce the Sphere, can they pay the extra cost? I think not, since Workshop reads "Spend this mana only to cast artifact spells," but I wasn't sure and wanted a second or third opinion.
Thanks!

[BC]

You are correct.

[brokenbacon]

Thanks a lot!

[LotusHead]

the shop mana (and extra mox mana) would pay for say, thorn of amethyst x2, which is different than the spell pierce (pay 2 more for spell pierce or else).