COE renewal prices are the same for Normal Cars and OPC cars.
The Revised Off-Peak Car (ROPC), Off-Peak Car (OPC) and Weekend Car (WEC) Schemes offer new and existing car owners the option to save on car registration related fees and road taxes, in return for reduced usage of their cars.
The OPC and WEC schemes are no longer available for registration or conversion.
Existing OPCs and WECs will remain registered under their existing schemes unless their owners apply to LTA to convert their cars to the ROPC Scheme.
Here's more info if you are renewing your car's COE: http://www.motorist.sg/article/120/renew-coe-vs-buying-a-car-which-is-really-better-for-you