A Raspberry Pi3 is the cheapest but in my opinion it isn't the best if you're looking to stream from your phone. There is a certain input lag from button to music playing, especially if using Airplay that drove me nuts.
I would seriously consider something from Auralic, Aries Mini would be a good budget friendly option.
I would seriously consider something from Auralic, Aries Mini would be a good budget friendly option.