The initial investment will be much less than a typical saltwater tank, but there are so many limitations it is almost not worth it.
Let's say you want a 5 gallon tank, that's about $10-20 for the tank, $30-50 for the live rock, $20 for the live sand, more for the lighting, etc.
And if you think you can settle for low lighting because you don't want corals, then what are you going to put in the tank? Very few fish (less than 4 small fish) can live there, so that just leaves you with inverts. Shrimp are fine, but filter feeders need pristine water conditions which are difficult to achieve in a small tank.
That's the same reason corals are hard to keep, the water conditions.
Now this was simply a warning, not a persuasion. I had a nano reef for quite some time, but maintaining the water conditions was very difficult and it became very expensive.
If you decide to continue anyway, I suggest checking out Nano Reef, they have some great info on that forum.
"Biology has at least 50 more interesting years."-James D. Watson