Don't know about other/better options, but it's about 46km from Louhu to Pingshan. Maybe OP should ask BYD whether they have a coach or car service.

From what I can find, the red/yellow taxis that can run inside the Shenzhen Special Economic zone charges 10RMB for first 2km, then 2.4RMB for each subsequent km. After 25km, there's a 30% surcharge for each subsequent km.

So, for a 46km trip, it may cost 10 + (23 x 2.4) + (21 x 2.4 x 1.3), or about 150RMB.

I'd think best option is get a private car with dual plates that take you from wherever you are in HK directly to the plant. A car/van service like this costs about HK$1,000 to SZX. This BYD plant is quite a bit further (and different direction), so expect to pay more.