A lot of detectorists own small weigh scales which can weigh items in ounces and grams. They are not expensive at all and are very useful for figuring out how much your your gold, platinum or silver finds are worth. Below is a link which will do all of the calculations for you. All you need to know is the weight and "K" values of your gold, which should be stamped on your jewellery.
http://dendritics.com/scales/metal-calc.asp
http://dendritics.com/scales/metal-calc.asp