Here is the deal. Any and every salt mix is made to mix into pure water. The salt mix already has all the components in it to turn pure water into synthetic sea water. Tap water isn't pure. Some facilities are closer than others, but none of them are pure. If you mix with tap water you are adding extra nutrients, chemicals, ect. into the mix which can have very different results depending what you started with.
Now RO is the best cheapest way to obtain pure water, that is why the majority of people use it. You could also use various filters to get the same effect, but you would be changing filters all the time and they add up. You could also purchase pure water but it adds up pretty quick too. Around here water is a dollar a gallon.
Best advise is to save up and get the RO unit. Its just one less variable to deal with if something goes wrong and you have to trouble shoot your tank.