The idea behind Spot instances is to allow you to bid on spare Amazon EC2 compute capacity. You choose the max price you’re willing to pay per EC2 instance hour. If your bid meets or exceeds the Spot market price, you win the Spot instances. However, unlike traditional bidding, when your Spot instances start running, you pay the live Spot market price (not your bid amount). Spot prices fluctuate based on the supply and demand of available EC2 compute capacity and are specific to different regions and availability zones.
So, although you may have bid 0.55 cents per hour for a r3.2xlarge instance, you’ll end up paying only 0.10 cents an hour if that’s what the going rate is for the region and availability zone.
Databricks uses spot pricing for Community Edition clusters to control costs. Click through for a very interesting discussion of spot pricing and how they take advantage of it.