It's about $50-$100 for the MOST basic model. Elaborate ones will run into the $100's. Not sure about the "UV Sterilizers" they offer at the LFS & if they are effective or a waste of money.
I think I've seen the LITTLE green machine if that's what it's called @ PS.
I too like to keep it NATURAL & simple whenever possible.
Definitely think you need more plants. The ones you are currently growing in your tank are SLOW growing varieties.
- Look for some red ludwigia so it adds a bit of color into your setup.
- When you buy the plants make sure you inspect, QT them for at LEAST a week & if necessary TREAT them so you don't introduce any UNWANTED hitchhikers.