The information supplied in bwight's answer is correct (+1) - here are a couple of details to back it up:
There is a respective FAQ at the the end of the Feature Guide: Amazon EC2 Elastic IP Addresses, see If an instance re-starts does the mapping of ElasticIP Address become invalid?:
A reboot or restart of an instance has no impact on the
mapping of an Elastic IP to it. However, terminating an instance will
remove the mapping and the Elastic IP will then be charged for each
full hour it is not mapped to another instance. [emphasis mine]
Spot instances are simply terminated, no different then terminating a regular instance yourself or being terminated by AWS due to a hardware failure, see point 2) in section How Spot Instances Work within Amazon EC2 Spot Instances:
If you’re running Spot Instances and your maximum price no longer
exceeds the current Spot Price, your instances will be terminated.
The charge only applies after the first full hour of being unattached, as detailed within the IP Information FAQ, see Will I be charged for the time when my IP address is unattached
because my instance failed?:
You are not charged until your Elastic IP address has been unattached
for a full hour. As long as you are monitoring your instances, you
will have plenty of time to reattach your instance before the charge
While this applies to a slightly different scenario and instances in general, the following FAQ hints on respectively identical behavior for your spot instance scenario, see Will I be charged if my Spot Instance is terminated by Amazon EC2
before the hour is up?:
No. If the Spot Instance is terminated by
Amazon EC2, you will not be charged for a partial hour of usage.
However, if you terminate the instance yourself, you will be charged
for any hour in which the instance ran.
I'd be surprised if AWS would apply a different pricing model for your scenario specifically, in particular given the comparatively small amount involved as well as the actual intention of pricing being unrelated to cost, see Why am I charged when my Elastic IP address is not associated with an instance?:
In order to help ensure our customers are efficiently using the
Elastic IP addresses, we impose the $.01/hr charge for each address
when it is not mapped to an instance.