You are on a tight budget but you are making it expensive for yourself. You are using 2 chips instead of 1. the opamp to create the ground reference and the isolator. Isolators are not cheap but it's your money you are throwing away. Learn this, engineering done to silly budget constarints at best just fails and at worse kills people. You do not have to use the current sense chip I suggested, there are loads of them and many cheaper than the TI one. If you think a programmer can design power electronics and signal processing then think again!
Learn about the building blocks and use them. Don't do the wrong solution just begcause it is the only one you know.
Let me guess, does the voltage regulation happen in the microcontroller by any chance?