I just spent the $60 dollars on the Instant Ocean big Salt box. It suppose to do 200 gallons of Salt Water. I do about 5 to 10 gallons water changes every week and I have had my box of salt for about two months now. The box comes with 4 bags of Salt in it, and I used the first bag up setting my tank up. I made 55 gallons of water with one bag using about 1/2 cup of salt per gallon.
The IO box has so much salt for mixing I don't think I will need to buy more salt for the next six months. It's differently worth the $60 (Might find it cheaper online) dollars it cost to buy the BIG box. They have a bucket that does 160 gallons but for ten dollars more I got the box and an extra 40 gallons of salt mix.
Also, I have read a lot of books and the advice they give you compared to the advice you get here can contradict one another. So you got to take what works for you, not to say both are not right. An example would be the Yellow Tang. A lot of websites say recommended tank size is 55 gallons and so does a lot of books. Some people here will tell you no tang should be in a tank that is only four foot in length. Some websites will tell you a 100 gallon tank minimal, but almost all the people I talked too outside of this forums says a 55 is fine because of their body shape not being torpedo shaped like other tangs are.
So like I said, you got to do what works best for you and your set up. You will get a lot of advice, and 90% of it will be good advice! I had so much help setting up my first 29 gallon tank, and than transferring it over to a 55. Because of this website and the information I researched my tank is successful (So Far).
When it comes to Salt, it goes without saying that it depends. If all your doing is a FOWLR
tank than the cheaper stuff at pet smart might be ok, but if you are doing more and a reef than I would go with a better brand.
Same thing with what water to use. A lot of people I know use their tap water and has had their tanks set up for YEARS without any issues. I myself use Reverse Osmosis water that I buy at my local Kroger and I than mix it. It's cheap and I think it has really helped my tank! I suggest you do the same, just get yourself some 5 gallon buckets from your hardware store and get your water at your supermarket. I pay $2 dollars for 5 gallons of R/O water.
Also, I don't know what size tank you are planning, but remember Bigger is easier! I wouldn't do anything smaller than a 29 for your first Saltwater tank, a 75 gallon would be great, and really not cost too much more. I say I spent about $1200 for everything when I started my 29 gallon, and if I had to do it all over I would have started out bigger. :-p Most of the equipment I bought can handle a tank higher than a 75, the only thing I would had to spend more money on would have been a bigger filter (I would do sump tank now that I know they are better), and more rock. I bought 55 pounds of live rock for around $260, for a 75 I would
only have spent around 100 more to get 75 pounds. My LFS
will sale their rock at $4.99 per pound when you buy more than 50 pounds at one time, I don't know if any other places do that. Overall I think I would have only spent about $300 more for a 75 vs the 29 I set up. Would have opened the door for a lot more options of fish. My 55 opened the door for me to have 3 more fish than the 4 I had in my 29.
Sorry for the long post. :-p
Good luck and have fun!