Here is some data I collected. See if it helps you.
PIPING ESTIMATING DATA
The following data is a summary of the averages of actual data from 4 representative projects conducted in the late 1990's. The smallest was $9,000,000 and the largest was $60,000,000.
Average Total Project Hours = 126,055
Average Total Piping Hours = 33,810
Average Percent Total Piping Hours vs. Total Project Hours = 26.82%
Average Project Line Number Count = 391
Average Project Isometric Count = 1,403
Average Total Piping Design Hours = 26,190
Average Total Piping Material Engineering Hours = 1,854
Average Total Piping Material Control Hours = 800
Average Total Piping Stress Engineering Hours = 1,528
Average Total Lead Piping Engineer, Dep't Prorate & Admin. = 3,438
Average % Piping Design Hours vs. Total Piping Hours = 77.46%
Average % Piping Material Eng. Hrs vs. Total Piping Hours = 5.48%
Average % Piping Material Control. Hrs vs. Total Piping Hours = 2.37%
Average % Piping Stress Eng. Hrs vs. Total Piping Hours = 4.25%
Average % LPE, Dep't/Admin. Hrs vs. Total Piping Hours = 10.17%
Average Total Hours Per Line Number = 86.47
Average Total Hours per Isometric = 24.11
You need to think about these kinds of activities, and time requirements for your company and project. Then you would now need to use your own labor rates to get a project price for piping engineering and design.