In my previous post on similar topic, the most cheap reply is around $1500 from HK Island to SZ Airport.  However I just find out that 中港跨境客運包車服務 is more cheaper.

cheaper will be from hk to sz then to airport

agree, take train, then airport shuttle

Just take the shuttle bus from MTR Kowloon Station (i.e. Elements shopping mall).  Less than 1.5 hours to the SZ airport directly.  ~RMB100 per person one way.

very frequent, but don't remember exactly.

It's either every 30mins or every hour.