My October 17 built GTD has the lowline camera fitted. I specced this as a standalone option and don't have Park Assist. The cost at the time was £265 from memory, however VW increased the price before it became standard.
The static green lines are pretty useless, however the camera still offers greater rearward visibility than relying on rear view mirror and parking sensors so I don't regret speccing it.
If all you want is as a visibility aid, the lowline version is fine, however as a parking and manoeuvre aid the highline is better. Whichever one it is though, I don't think £340 is too bad a price for a retrofit, as long as they're not using low quality parts.