Your best bet is do what I did. Stalk Craigslist for a couple of months and find someone who is getting out of the hobby. I happened to find a $150 deal which included the tank, lights, skimmer, test kit and a heater. It's a 56 gal
tank. Some say it's small but for me it's perfect. I buy small fish and sort of watch them grow. Anyway on top of the tank you'll need a filter. I bought a fluval 406 for 160 bucks, again some say canister is wrong for saltwater but my fish are fine. I do 5 gallon water changes a week and I thoroughly clean my filter once a month and put two new bags of carbon every 3 months. I cycled my tank with 4 damsels and it's the only thing I regret. None of them died and they seemed happy throughout but it takes way too long to see progress. Mine took about 8 weeks and I still was never sure that it was cycled. I lost a long nose butterfly in the process. I was so anxious to put fish in the tank after I started it but if you just suck it up for a week and put a piece of shrimp in the tank to cycle it you'll get to put the fish that you really want in your tank sooner. And you'll know that it's definitely cycled as it is much easier to track the progress with real ammonia source.
So as far as the 400 dollar budget goes, it's definitely doable to get your started. But it's such an addicting hobby that once you get fish in there you'll start searching for ways to provide luxury for them and start justifying the need to spend more money on them. They definitely deserve every penny because they're gorgeous.
If you don't want to spend money then I suggest you stop reading now. Stop going on forums, stop looking at pictures of fish and find another hobby because the more you read on the more you'll burn a hole in your pocket. Speaking from experience once you say you want a saltwater tank and you walk into a LFS
it's pretty much too late and you're helpless. Sorry
we're all hooked and you are too.