Wawo via Amazon offers the Wawo 10" Bluetooth Keyboard Case for Samsung Galaxy Tab 4 for $27.99. Coupon code "LOVEWAWO" drops it to $22.39. Plus, Prime members bag free shipping. (Non-members can spend $35 to bag free shipping; otherwise, it adds $8.54.) That's the lowest total price we could find in any size by $2. It fits 10.1" tablets.
A close price: Btocfactory via eBay offers it for 8.4" tablets for $22.77 with free shipping.