In my opinion the easiest way to start your online business is email marketing, all you need is:
Squeeze page: You cab get one done in fiverr for $5
Lead Magnet: You can get a report done for you in fiverr as well for about $20
Domain name: you can get one in go gaddy for $1 (that's for the first year) just get yourself a coupon.
Autoresponder: there are many out there like aweber, getresponse, sendlane, etc.
Hosting: there are cheap options like hostgator for about $7/month.
As you can see, you can start your online business very cheaply and without problems, all you need is about $33 USD and that's for the first month, after that it can go down to just the cost of your hosting.
Now.. all this is having into consideration that you will not pay for traffic, and only spend time in order to get the traffic you need to start getting subscribers.
If you want to invest in traffic then to get started you would need $150 aprox., you can get cheap traffic in clickonomy.
Hope this helps you get started
To your success,
Jonathan Nunez