With only two previous printings, we can expect a meaningful drop in price, likely between 30–50% in the weeks following the release of HOC. This piece of equipment is particularly sought after in Commander, where protecting key legendary creatures is crucial, and sees some play in casual formats as well. The demand in these formats may help cushion the price drop somewhat, but an increase in supply will inevitably affect its market value.
If you've been wanting to add Mithril Coat to your collection or deck, it would be wise to hold off for a few weeks after the set's release to take advantage of the anticipated price decline. On the other hand, if you currently own multiple copies, it might be a good idea to list them before the market sees an influx of new stock.