basically youre gonna want to buy a small digital scale, because no smoker should be without one. You can get these fairly cheap at places like walmart.
thats pretty much the only way you will know for sure whether your are getting ripped off. You should also test the accuracy of your scale before each use...a nickel weighs exactly 5 grams, so if your scale says differently, its wrong.
Generally, an eighth of nugget should cost 50 dollars, and no more. eventually you will get this for a cheaper price, because you will meet people higher up on the "corporate ladder" An eighth of nugs should weigh 3.5 grams.
If *yuck* you are smoking shwag, it shuold cost no more than 20/eighth, but this shuold weigh up to 5 grams, to compensate for the seeds, and shittyness of the pot.
ONe thing you need to remember is that if you are buying from somebody who is not a dealer, some sort of tax should be expected. the person who is getting it for you, if they pay retail price, shuold get somethign for it. my suggestion is that you find yourself a dealer, who will sell you straight shit for regular price or better, but until then, buy a scale and weigh your bags (always outside the baggie, because they vary in weight from .7 to 1.4 grams), and tell your friend that you wont buy the bag unless its straight, but you will smoke a bowl with him, or throw him a couple of extra dollars.