It's currently marked down to $3.39 (66 percent off), which is a price worth jumping at. There's also a four-pack available for $10.19 if you can convince others to split the cost, or you just want to give out extra copies to nice people in your life.
It's a good thing the sale is happening today and not earlier in the week. Risk of Rain is one of those games that can make the world around you fly by. Best played on weekends.