If the surviving wife wishes to sell the property during her lifetime, the best method of allocating the sale proceeds between her and the son would be to use the valuation tables published by the IRS. The allocation established by these tables changes continuously because of interest rate changes in the economy and because of her increasing age.
Assuming there is no will, then with regard to the homestead, Minnesota Statute 524.2-402 provides that a surviving spouse, who does not have children with the decedent, gets the house for her life (which she could rent out or use the property), and then, upon her death it would pass to the decedent's heirs. She cannot be forced to sell the property.
I agree with the previous answers. To sell the property in most cases the life estate owner has to agree with the owners of the remainder interest (the interest in the property after the life estate owner dies). So since both sides have to consent, they are free to come up with any split they can agree on.