Thanks for all your replies.

I already had a spare computer, so I ended up buying a Radeon RX480 to mine altcoins and exchange them in Bitcoins.

A RX480 (~200€) with the rest of the computer consumes 240 watts and can "mine" 0.008 to 0.009 BTC per day.
If I went for an Antminer S7 (~350€) and underclocked it, I could of mined 0.00072117 BTC per day (1.6Th/s @ 400 watts).

I ended up spending less, using less power and getting more BTC per day.