The "Check Current Spot Price" button in the configuration does a request to AWS to retrieve the spot price history of the specified instance type but it doesn't set the "Product Description" and displays the first price returned. This can differs greatly from the actual spot price expected to be returned.
For example, a request for the spot price at the moment returns:
I'm interested by the "Linux/UNIX product" (normal on-demand price is $0.3880). The current spot price for this type of product is $0.1417 but the current price reported by Jenkins after clicking on the button will be for "SUSE Linux", which is not as interesting.
In some cases, the spot price displayed by Jenkins is actually higher than the on-demand price for the product we are interested in:
(displayed spot price is $0.1161 but the on-demand price for "Linux/UNIX" is $0.0536)