first off, welcome ot the club. second, you are going to get many answers/opinions to your questions. Best advice you could receive is do a lot of research and search the forums here. You will find many of your questions have been answered in the past, and if not, ask away, there are lots of knowledgeable people here.

As for cost, it can be as much as you want it to be or as little as you need it to be. Ive got a pretty simple setup that is pretty maintenance free and I dont have a lot of money tied up in mine.

As for the saltwater you can buy it from a LFS or you can mix your own. In the long run making your own would likely be a lot more advantageous. You are correct though, tapwater will likely keep you battling algae for ages but an RO filter can be bought pretty cheap and will save you money in the long run. water changes, once again you will get lots of opinions but I have a 37 cube setup and only do a water change about once every two months and typically the bigger the tank the more room for error (but that is not saying much)