Hi war relics,
If I understand well, the purchaseprice is the sellingprice with tax.
In HikaShop you can configure many different tax rates, and for each products, you can choose the tax category of the product.
Depending of its category, you can set the purchaseprice or the sellingprice, and the other price will be calculated automatically.
On the front-end you can choose to display the price without tax, with tax or both.
And in the back-end you can see both prices.
You can see some documentation about taxes here:
http://www.hikashop.com/en/support/documentation/37-hikashop-taxation-listing.html