With orders of $25 or more, Amazon offers Prime members the Nalgene 32-oz. Tritan Narrow Mouth Water Bottle in Clear, model no. 3420-NMWB, for $4.50 with free shipping. (Non-members can spend $35 or more to qualify for free shipping.) That's the lowest total price we could find by $6. It's BPA-free and features a loop top.
Note that this item is part of Amazon's add-on program and can only be purchased with an order of $25 or more.