Is the lower unit price always the Best Buy

No.

Other factors come into play, including product quality, product life, resale or salvage value, aesthetics, safety, need, environmental responsibility, suitability for the need, and more.

A truckload of soap may have the lowest unit price, but it would not be the best buy if you only need one bar. A bicycle may have the lowest unit price, but will not work as a tow vehicle for a 3-ton trailer.