In 2015, the exchange rates had reached a new low of around $200 per coin. The historical peak at that time was $1,231 per coin which was attained in 2013 December. So the prices in 2015 represented a 85% decline from the peak valuation. Going by the same logic, an 85% reduction from the peak in 2017 December ($20,000 per coin) comes to around $3,000 per coin.