Evaporation isn't a good option since it costs too much energy to heat the big amounts of water.
Is it? I've seen prices around USD 130-150 per ton (minimum order of 25 tons) for ammonium sulfate with a purity around 99%. You might want to calculate the costs to operate a production plant using natural gas (or other fuels) for evaporation yourself.
There is however a different energy source that you might want to take into consideration: the sun.
Evaporation of water from solutions of sodium chloride in open ponds by solar heating alone has a long tradition in the production of sodium chloride in the form of sea salt.
Ammonium sulfate will not evaporate from the open ponds. At temperatures between 235-280 °C, it will start to decompose.
However, if surface temperatures reach that level under irradiation with unconcentrated sunlight, the fate of the ammonium sulfate is the least of your worries ;)