How to buy fish sauce
For those of you not familiar with fish sauce, I know what you are thinking: what a terrible name. I have opinions about branding (or lack there of) of Asian products, but in short I can't emphasize enough how the name "fish sauce" does not do this staple justice. It is magic. It is what you need to cook Vietnamese food (if you aren't a vegetarian). Think of it more as it's main ingredient: anchovy extract, which is the same ingredient that makes worcester sauce and bloody marys so good.
The quality of fish sauce that you choose definitely affects the price, which is why there is such a large range in price. In my family we usually buy two types - one for cooking (cheaper), and one for dips/vinaigrettes (expensive). Here are the three brands I would recommend depending on your budget and how frequently you plan to use it:
Squid Brand (cheapest) - a solid and inexpensive option, it is what I mostly grew up eating.
Viet Huong (moderate) - a slightly higher quality product, and you'll notice it more if you use fish sauce in dipping sauces and salad dressings.
Red Boat (pricey)- the Ferrari of fish sauces. It is pretty pricey (can be 10-15 times as much as Squid Brand!) but my oh my it tastes great.
At home these days I usually use Squid Brand for cooking (soups, marinades, etc.), and Red Boat for my dipping sauces that will be eaten raw. Though I recommend buying the middle option if you have trouble committing to so much fish sauce eating :).