What should I accept when selling my portion of the deed?

Legally, you probably did not sign under duress: you made a bad decision under pressure, at a bad time of your life, but legal "duress" is generally more than pressure; it is a threat of something itself illegal, like violence or a criminal activity. While it would be worthwhile for you to consult with an attorney in detail about this, to see if their behavior did rise to the level of duress or if there is some other reason to void the sale--e.g. fraud (if they lied to you about something material or important) or lack of capacity (if there was a medical psychiatric diagnosis that at that time, due to grief or other factors, you had diminished mental capacity)--there is a good chance you cannot undo the transaction.
However, they *cannot* legally make you sell your share of the home to them or otherwise, unless they bring a legal action "for partition" (see below). Therefore, you can hold out for *any* amount of money you deem fair--such as, for example, that they pay off or refinance the mortgage to remove you from it (otherwise, you'll still be liable under it; do not sell if the mortgage is not taken care of) and otherwise pay you, say $37,500 (half the purchase price, less half the mortgage).
An action for partition is a kind of legal action (lawsuit) in which one owner of a home asks the court to order that the home be sold and the proceeds split among the owners, because the owners are at loggerheads and can't decide what to do with the property. If a court were to grant the order, the home would be sold, the costs of sale and mortgage paid from the sale price, and you and they would split what is left.