Having them replaced by just anybody could be a waste of money, and you could end up ingesting and breathing more mercury.
It's hard to put a cost on dental work not done right. IMO you are better off spending the money and having it done right.
I had a root canal tooth removed years ago by a regular dentist. About a year and a half ago I had to have a cavitation done on the same area where the tooth was removed because the dentist failed to clean the socket out properly. The extra cost to do it right, about $1500 in addition to what I spent originally.
They had to lay the gum open and clean the socket out with a burr. The jaw bone was like mush from the bacteria that the first dentist didn't clean out when he removed the tooth.