Similar question was posed on reddit a few years back. This answer seems the most pertinent:
"You could run or schedule an Item Price Mass Updates every day or whatever. Go to Lists> Mass Update> Update Prices. On the Mass Update page you could select _Average Cos_t, for example, in the Bases On dropdown. Enter 30%, or 25%, etc, in the Amount field. You'll have to set one up for each price level but you can run it automatically on a schedule or on demand. "
In your case you can base it on the most recent cost.