If you truly can buy at 5 (wtf is that? 5 euro per what?) and sell at 135 (again...135 what for what unit amount?) - assuming you use the same units and buy at 5 whatevers and sell to grid at 135 whatevers.....then why would you mine at all?  Just buy at 5 and charge capacitor banks, batteries, anything you can connect to store grid power at a cost of "5 whatevers", and then immediately dump those saved charges back to the grid at "135 whatevers."

You would get a continual 27x on your money just by charging capacitors then discharging them into a grid-tied inverter.  No mining neccessary.

Therefore I question your "buy at 5, sell at 135" rate.