I recently came across the Hailea FA400 tank which is really a freshwater tank with saltwater pretensions - it comes with a blue T5 tube alondside a white one! Although it is clearly limited in what it will support, I saw a little potential in it and have modified it into something that might actually work, for a cost less than $500 (although there are always optional upgrades). It is a basic curved glass tank with a false back wall a-la RSM. Unlike the RSM (or maybe like the RSM) there is no room for a good skimmer, so this setup will only work with light stocking, regular testing, and religious water changes.
Here is what I did:
Step 1) Cut a small overflow in the back of the tank and block the lower intake with a piece of plastic.
Step 2) Remove some of the stock sponge and refit with a bag of charcoal and 100watt heater
Step 3) (optional) Replace the stock 380litre/hour pump with an Eheim Compact 1000 (it will just fit if you flex the rear wall forward) and drill the spray bar with a few extra random holes to create more random currents. I've also drilled a couple of small holes on the back that bounce water off the back wall.
Step 4) Build rockwork from coral rubble using Aqua-Kneadit. The goal is to have rockwork that is aesthetically pleasing in itself - you won't be hiding it under lots of coral, so it has to look good to your eye 'naked'. I've made a little cave suitable for a firefish etc, and tried to create little platforms a few centimetres below the surface so that the corals I have used receive as much light as possible
Step 5) Fill, add a handful of live sand, and let the tank run in. Random bugs will appear
Step 6) Add herbivores (snails, little starfish etc etc, or in my case a Sea Bunny) and a few corals that you know will tolerate relatively dim light high up on the rockwork
Step 7) In a week or two, I will add a fish - probably a Bangaii Cardinal.
Basic cost:
Tank $149.90
Heater $64.90
Charcoal and Bag $28.90
Coral Rubble $28.00
Coral Sand $24.90
AquaKneadIT $14.90
Salt $39.90
Total $351.40
leaving about $150 for stock, enough for a fish or two, or a fish and a shrimp etc.
Optional upgrades:
Eheim Compact 1000 $79.90
Pro Salt (contains calcium) + $40.00
Purigen $34.90
Anyway, cheap enough to give marine a crack provided that you realise that this is the bare bones minimum to keep some basic low-light corals, a small fish and perhaps a shrimp or something - no more - and that you will need to do regular water changes. Have fun