Saltwater and Freshwater Tank Owners- Cost Estimate?

It really depends on how big of an aquarium you plan to get. Of course the bigger it is, the more expensive it will be. When you get a bigger tank, everything else will have to be "bigger". Including the filter, heater, air pump, decoration, everything.

Just what size of a tank do you wish to get? Glass or acrylic? I can't really give an estimate without knowing what you want.

My main tank is 40 gallon acrylic tank, with a canister filter. The tank itself was over $200. A glass tank of the same size would be cheaper but easier to break. Canister filter is also the high end of the aquarium filter systems, which makes it more expensive than some other types of filters.

For details on how much total cost on my aquarium, you can check out the link in my signature.
Yes, I spent a lot more than the average cost others would have spent on a tank of similar size.

As for saltwater tank, while I have no experience with it, I can tell you that it will be more expensive than freshwater tank to maintain. I wouldn't recommend beginners to go straight for a saltwater aquarium before having any experience with a freshwater aquarium first. It is a lot more complex.