The dirt cheapest way to dechlorinate is mixing your own dechlorinator using sodium thiosulfate. The only problem is with chloramine, it will leave you with ammonia, which can cause problems depending on the amount of water changed, level of disinfectant, and capabilities of the nitrifying bacteria in your filter.
I'll bet chlorine does do a real number on marine fish. They are from what I understand quite a bit more sensitive to environmental changes than most freshwater fish. While I have never done a marine setup, I do know that you shouldn't start with tap water, even if it is dechlorinated, and changes in temperature that would be fairly routine in a fresh water setup would be bad news in a marine tank.
Many old school fishkeepers still don't use dechlorinator, don't do more than 25% water changes, despite the fact that the water they think contains chlorine also contains chloramine now. They do a good job breeding & keeping fish, thinking the chlorine is just gassing out, and a bit of disinfecting is good for the health of their fish. Trying to teach them the science behind it is akin to trying to teach my dogs to read, they know what works in their setup, and aren't too concerned why.
While this works for seasoned veterans who remember when hob filters were a newfangled gadget, it is in no way the best starting point for a new aquarist. This is why from the beginning dechlorinating water is advised, water testing, along with many other things that add up to making starting out less problematic. Most everything concerning aquatics is guidelines, there are few carved in stone rules. Once you understand why these guidelines are suggested you can start to stray outside of them, hopefully with a solid backup plan in place.
