I think you are throwing money everywhere without much thought. If you target a MNS and buy a full SEO service, I'm pretty sure you'll rank somewhere in the top 10. Maybe you should re-research you keyword and see if you chose the right one. With over 500$ invested, you should atleast have made half of that back.
Anyways, keep us posted and good luck!
Anyways, keep us posted and good luck!