PricePlunge offers the V7 Slim Clear Case for iPhone 6/6s, model no. PA20C-CLR-47-14N, or iPhone 6/6s Plus, model no. PA20C-CLR-55-14N, for $5.99 with free shipping. Excluding the below mention, that's the lowest total price we could find for either model by $2. It features TPU / polycarbonate construction and a 2-piece design.
Of note, with orders of $25 or more, Amazon offers the iPhone 6/6s Plus case for $5.55 with free shipping for Prime members. (Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.)