Ye, unpaid is always a bit tricky. For a large map like this, you are easily looking at 400-500 dollar bill to get it done professionally (or even more than that, depending on who is the artist). I get that if you are just running games for your friends, paying that much money can seem way too much, but sadly for the artist the time and effort needed to complete the map is exactly the same than it would be if you wanted to use it commercially.