I tried configuring the regular phat to a 128gb ssd and 8GB RAM to match the rMBP. It turned out to be even more expensive.
As the 13" Pro isn't designed to be a gaming machine, I seriously doubt regulars would use more than 8GBs of RAM.
How would it be more expensive? I have checked online other retailer are selling 128gb ssd at an average price of $160 and 8gb RAM i am not sure how much it cost. Probably less than $100?