My mother recently passed away and I am looking to repay her debts. She did not perform prayers for 3 months while she was in hospital due to her being unconscious much of that time I think she may have also had a period of time much earlier in her life when she did not pray on a regular basis. Can I pay sadaqa as kaffara for these missed prayers? How much should it be for each day / prayer ?

There is no expiation for the prayers your deceased mother missed while being unconscious in the hospital nor for the prayers she missed earlier in her life. This is because she may have made up what she missed during her lifetime or have started making them up. Therefore, she will not be blameworthy. You should rather pay charity on her behalf as whatever you wish, ask Allah to forgive and have mercy on her and know that Allah is the Ever Most Merciful upon her.