it's hard to tell exactly what you would call a 30$ sac, some people sell half Os for 25$ some up to 35$. If you trust the dealer then don't worry about it, but if it's a new connect then it's hard to tell. Like other's have said buy a small scale, prices range from 5 to 7$ depending on area and outlet source. Personally i wouldn't buy pre grinded weed, buds are better but at the same time i have gotten shake sacs from people i know that won't rip me off. Sometimes we got to get what we can.
If you aren't sure it's right and your gut says so then it's probably not. With out a pic it's even harder to say if you got your $'s worth.