My laptop had at the moment only 4GB of RAM. Windows 8.1 and intel core i 5.
What would it cost if I upgrade to 8 or 16 GB RAM (RAM)?
First you have to see if it is possible with your laptop, that is not in each case the case. Then you have to see how much you can upgrade and what ram it is. Only then can you talk about what it costs
So if you want to get it upgraded, you need to buy an 8GB Ram Stick first, and if that costs me to understand it, upgrade it to 20 euro.
But look s.besten in your laptop description, if you can expand it at all, but I guess it is up to 8GB possible.
But you can easily install it yourself.
'Good' 8GB Ram Sticks start from 50 euro.
Good 16GB Ram Sticks start from 110 euro.
You just go to a PC workshop and let you advise there. They also create an offer.