I bought my saltwater master test kit ( ph, ammonia, nitrite, nitrate) for about $30 at my LFS
. Depends on what you need - they also have a reef master kit with phospate, calcium etc. Check out Drs Foster & Smith or Big als if your LFS
isnt decently priced. They usually have good sales and decent shipping. You might also want to keep an eye on petsmart.com, they're only have basic stuff, but they do free shipping over $50 or $70 ( if you need more stuff as well) every once in a while.
A good plus to all these kits is that they come with intructions for each individual kit about how much water to use, how many drops and scales for measuring color.