Loud knocking like a rod

About a week ago I was driving my Mustang gets a
convertible with the six cylinder 3.8 engine in it has
81,000 miles on it I got it up about 9 you started
making a knocking sound so I slow down and later
check my oil chillers and just a little bit low. The
next day when I go it when I first started division
knocking in the guy holler at it started to get louder
I then had a couple guys listen to it I told me that I
might be a lifter or a main bearing but they didn't
think it was anything serious yesterday started
knocking in loud enough that I had to shut it down
for Freight I'm going to completely threw a rod or
do some very serious damage. It only has a 1,000
miles on it and is always been well maintained
what would this probably be and how much is it
going to cost me the car is in Immaculate
condition it looks like it was just driven off the
showroom floor