Law prohibits you from selling goods above MRP.
So you cannot sale above MRP unless there is some exception (example, water bottles in hotels).
Tax paid under RCM will never be cost as you will get full input tax credit. How you calculated 180 as cost? You will get input of Rs. 180 regardless of at what price you buy pen.
Though trading at MRP to MRP will not bring any profit but there will be no tax loss.
You will sell at MRP including tax. Here your loss will be actual cost of 1000 Rs. minus at what price you sell without tax.
You can utilise input against other sales.