I spent between $600 and $800 initially on my mini m. I did buy ADA CO2 and lily pipes and a drop checker though which added to the cost along with the matching light.
$300 can get you a start but you'll probably be paying more once it is fully set up. Realistic price is probably around $500. Obviously it depends on your light, filter, pipes, and co2 setup.
See what else am I thinking on facebook.
Rumford Aquatic's Tools selection. Best prices anywhere. Guaranteed! Every purchase supports me an entrepreneur studying at ASU.