A high demand increases prices when the supply cannot satisfy all requests. This might lead to some people taking advantage of the situation. Aside from asking to connections you might have in Berlin, you can also ask for advice on the Facebook groups I mentioned in this post to double check if the price is all right. You can also refer to this map made by Immobilien-Scout & WG-suche for guidance. Based on their data, an average room costs 429€ in a flat share. It fluctuates on your location of course as you can see on this map.