The cost to build an Online Business or Web Application varies based on how complex your idea is and whether you need to build it custom or using an off the shelf open source software.

Building a custom web application provides better flexibility in customizations but may cost more, but using off the shelf software provides less flexibility in customizations and someone else can easily copy the same idea by using the same off the shelf software.

I have seen custom web applications cost anywhere from $20K to $100K and building web applications based on off the shelf software anywhere from $10K - $50K.