First, let me welcome you to apc. Yes, you can do it a lot cheaper than $4K, though, imo it will cost you more than $500, unless you go 'garage sale' route.
I've never kept a salt water tank, but maintenance can be as little as topping due to evaporation if you go the 'el natural' type tank, to weekly maintenance where you do partial water changes/re-fills, or daily to tweak and prune and meticulously maintain, if you're so inclined.
Here's a couple of references which might help you:
http://www.aquatic-plants.org/articles/basics/pages/index.html
http://www.rexgrigg.com/
I've never kept a salt water tank, but maintenance can be as little as topping due to evaporation if you go the 'el natural' type tank, to weekly maintenance where you do partial water changes/re-fills, or daily to tweak and prune and meticulously maintain, if you're so inclined.
Here's a couple of references which might help you:
http://www.aquatic-plants.org/articles/basics/pages/index.html
http://www.rexgrigg.com/