I do not believe you shall find a taxi that would be travel from Zhuhai to Zhongshan and if you do, the taxi fare shal extrodiarly high since the taxi must also consider for him/herself an additional return fare for the taxi to return to Zhuhai, which is typically not an issue for local routes where a taxi could pick-up a new fare after dropping you off.

The only option would be a long-haul bus line. You would need to goto the mail bus station and ride a station-to-station bus at around 40 RMB oneway.

it's around 50 CNY to 100 CNY.one way.