Q: Hello, Dave I have a 2006 mercury mariner it just started as of last week releasing thick smoke from the exhaust I'm no profession

Hello, Dave I have a problem. Just as of last week my 2006 mercury mariner had started releasing thick smoke from the exhaust and been kind of hard to gain speed when I accelerate however I am no pro at cars by far and I've asked around and was told by one mechanic that it's my head gaskets they may be blown it's thick smoke coming from the back of the truck I was told that it could cost up to 3700 dollars to repair this .. Can you give me an ideal of what I should pay to have it fixed due to the fact I've searched the internet and seen that the parts are not to expensive and checked a few labor fees out and it wouldn't mount up to what I was told before the price of 3700 is really steep !! what can it really be if its not the head gaskets thanks in advance..

My car has 251000 miles.
My car has an automatic transmission.