Nobody seems to have mentioned the difference in the engines.
The Standard Octavia has a 1.6 or 1.8 TSI (118kW) or a 2.0 TDI (103kW) choice.
The RS has a 2.0TSI (147kW) or 2.0 TDI (125kW) engine choice.
There is a major part of the cost differences in these engines and the associated plumbing and hardware that goes with them.