This paper investigates cost reduction potential for thermocline filler (TCF) storage in a direct molten salt parabolic through plant based on system-level simulations in response to weather data for a reference week and corresponding plant control. In a first step the theoretical potential for investment cost reduction of TCF tanks filled with nitrate salts and natural stone as filler material operating in a range of 290°C and 550°C is represented. The effective potential on system-level though is reduced due to a temperature drop at the end of the discharge cycle. Therefor the electricity cost for selected reference days is chosen as a measure for the economic potential of TCF systems over two tank (2T) storage systems. As a base case a 100 MWel direct two tank parabolic trough power plant with 13 hours storage capacity using molten salt as heat transfer fluid and storage medium was chosen. This setup is modelled and simulated as a function of TES capacity and solar field size using the commercial software EBSILON Professional©. Based on best and worst case assumptions for the financial parameters, a total TES investment cost reduction of 30 to 42% can be achieved and a first estimate for the levelized cost of electricity (LCOE) results in 5 to 9% improvement as well as an increased capacity factor and a higher storage capacity.