We have a 45 gal and a 55 gal tank. We have tropical fish in freshwater, not salt water. We bought the 45 gal tank - fish and all - from someone on criaglist. It cost about $100. We did replace the filter because it looked pretty old. We really enjoy it.
We keep aggressive fish (e.g., sharks, tiger barbs) in one tank and community fish (e.g., angel fish, neons) in the other tank. None of these fish are very expensive, so if they kick the bucket (or the tank), they are not expensive to replace. Yes, the saltwater fish are prettier, but you can get some really pretty fish for the freshwater tank.
It does take quite a bit of work to keep the fish and environment healthy. We just had a spontaneous overgrowth of algae and will have to completely clean everything, rocks and all to get the algae off. Oh and the tank we bought on craigslist had snails in it and we battled them until we learned that clownfish will eat snails. Two clownfish later, we've had no snails in over a year!
My hubby has spent lots of time at the fish store getting tips on how to maintain the tank and solve problems.
Our son-in-law gave us the 55 gal tank because he was tired of it. So we lucked out on the set up with both of them.
We love both tanks and after the initial set up, they aren't that expensive to maintain - it just take diligence to keep everything healthy.