What is the cheapest card to send text messages on the mobile?

I think the Green Campus Package in the M-Zone should be the cheapest way to send short messages. Most students are using Qingqing campus now. They rent 300 messages a month in 20 yuan, 500 messages a month in 30 yuan and 0/000 in 50 yuan/kloc. The higher the monthly rent, the more information they leak. If most of their friends are mobile, I think this is very suitable, but it costs 0.65438 to send to Unicom and PHS.