From Dario Fanjul:
Price Contrast allows you to determine the lowest price in terms of price/quantity. Just enter the items and Price Contrast will show you the price per unit.It is possible to compare items with different units as long as they belong to the same category. At the moment there are four categories of products to compare: Unit, Mass, Length and Volume. Supported units for each category are:Unit: just units. :)Mass: dram, grain, gram, kilogram, milligram, ounce, pennyweight, pound, ton (UK), ton (US short), tonne.Length: centimeter, decimeter, foot, inch, kilometer, meter, mile, millimeter, yard.Volume: bushel (UK), bushel (US dry), centiliter, cubic centimeter, cubic decimeter, cubic foot, cubic inch, cubic meter, cubic millimeter, cubic yard, dekaliter, fluid dram, fluid ounce (US), fluid ounce (UK), gallon (US), gallon (UK), gill, hectoliter, liter, milliliter, minim, peck (US dry), pint (US), pint (UK), pint (US dry), quart (US), quart (UK), quart (US dry).Units for each category can be enabled/disabled to see only the units you want.