How to determine actual cost of specific spot instance
From Amazon EC2 Spot Instances Pricing:


  The following table displays the Spot price for each region and
  instance type (updated every 5 minutes).


So one (crude) option would be to parse that page to get the price matching your particular instance type and region.

I don't know if the equivalent info is available via the AWS Price List API:


the Bulk API clearly doesn't support it:



  Offer files do not include information about expiring free tier offers
  or Amazon EC2 Spot Instances.



the spot instace pricing isn't explicitly declared as unsupported in the Query API, so there might be a chance for another option here, that'd be better than parsing pages. But I didn't try - I'm not an AWS user yet.