You can't predict the full price before you pick date of the return flight and actually change the booking.
At time of booking your return, you can go for the lowest available fare class, but there is not guarantee that it's the same fare class that you originally had. In fact, it's unlikely since there are so many fare classes.
The total cost of the change is "difference in ticket price" + "Change Fee". Note that "difference in ticket" can also be negative, so you could get a refund to offset some of the change fee. That doesn't happen very often though.
It may be worth looking into the price of a "flex ticket" which has a reduced or no change fee. Typically these are significantly more expensive, but sometimes there are about the same price and I have recently booked a flight where the flex ticket was cheaper than the regular one. So you never know, and it doesn't harm to check. Since you already know that you will change, a flex ticket would be better if the price difference is less than the change fee.
If you do book a "flex ticket", be sure to read the exact fare rules (or have an airline service agent translate them to you). Typically it's "no change fee but you are still responsible for any difference in ticket price", but that's not always the case.