How to sort product displays by price using search api sorts?
my solution is:


Fields Tab: in Related Fields section add Product-Price multi field
Field sTab: check field_product:commerce_price:amount_decimal for index (integer)
Filters Tab: enable aggregated fields. 
Filters Tab: add aggregated field: name - your custom label, type of aggregation - first, Contained fields - Price > Amount (decimal)
Fields Tab: check your new aggregated field for index (integer)
Sorts Tab: now your custom aggregated field available for sort


Be sure you are using latest dev version of the Search API sorts because of the https://drupal.org/node/2202093 issue.