Linda's remark at the play's end means that they're now free of debt; the mortgage on the house has been paid, evidently with insurance money from Willy's death. Ironically, of course, Willy isn't there to share this freedom with her. All their lives together, they have struggled to make ends meet; finally the family has some money but only because Willy chose suicide.
Biff's declaration to his father that they are both "a dime a dozen" means they are just average, ordinary men, not special in any way. Willy has insisted for years that he is a remarkable salesman destined for great success; likewise, when Biff was young, because he was an outstanding athlete, Willy had enormous expectations for his son's future. These hopes were never fulfilled. Biff wants Willy to see that they are not destined for greatness or special treatment; being ordinary is acceptable and normal. He wants his father to face and yield to reality so that they can get on with their lives. Willy, of course, is too entrenched in his view to accept Biff's statement because it completely contradicts the way Willy sees himself and his son. Then when Biff breaks down and cries, Willy is astonished and says, "--he likes me!" Willy completely ignores what Biff has tried to convey.
cybil's answer says about all that needs be said, but I have one observation. I believe that the house was not paid for by the insurance money. Most life insurance policies do not pay on suicides, and there is evidence that Willie may have tried this in the past. I'm pretty sure that an insurance company would pick this up. It's part of the irony of Willie's life. He couldn't even "end' his life as he wished.
And Linda's remark also has some ambiguity. Freedom exists on many levels; Linda is "free" in the sense of "out of debt." But she will never be free of the consequences of her life with Willie and of the life of Biff and Happy --- and Biff and Happy will never be free of Willie's influence on them. Somehow the past is always there ....
I like your last remark, Tim. Reminds me of Faulkner's famous remark, "The past isn't dead and buried, in fact, it isn't even past."