Discount v Energy - Table?

When playing I've noticed there are certain items of similar value which give differing values of energy when discounted, and certain items which have a gold value which is worth vastly more than the energy from discounting (Opulent items for example, considerably more gold, but much less energy when discounted than similarly priced items).

Has anyone put together a table on this? I'm trying to make the shop more energy efficient by stocking a core of items that are discountable for maximum energy at minimum level/cost, but finding it hard to make sense of the tables from St-Central to do it.