Derive Product sell price based on purchase cost

This needs a better spec. Also need to take in count the fact that these are aggregate (ie. multiple purchases of the same product with different price).

No files, tests, or entry points are named. Start by defining how sell price should be derived from purchase costs when a product has multiple purchases at different prices, then identify the product and purchase data paths. Done means the agreed pricing behavior is specified and implemented with coverage for aggregate purchases.

Written by the indexing model from the issue text.