Some SIM cards are preloaded with y50, y100, y200, etc. In such cases, you need to pay for this in addition to the card itself, but that's no big deal because chong zhi ka are rarely discounted by more than 1%.

Moving on, average street prices tend to be significantly lower than China Mobile offices. In fact, you can usually get a relatively good number for around y100, while bad and really bad numbers typically sell at a loss (iirc, I've seen Shen Zhou Xing sim cards for as cheap as y25).

I used to have a post paid number, which was cheaper on all fronts (and you can search through thousands of numbers), but those take around 40 minutes to set up and you need a local bank account as well.

Back to your question, like Jamoldo suggests, just say, "sim ka".