If I want to get a website build like elance where companies and users post their jobs looking for online bidders.How easy/tough it is going to be considering that its not any new idea.OR is it a very bad idea?
what could be the approximate cost of hiring somebody to build such a website, and is it going to be worth to build such a website for a start-up/newbie.
The costs depend on:
There are many questions open - you probably need an iPhone dev. How many customers to you expect? How good should you design be?
I guess you can have a team working with 2 java devs, one frontend/mobile crack and one designer/tester/ui expert + you. They could work a whole year on that without any problem.
But you can also do a very cheap variant with a php scripter who also does the design. He can have a nice outcome after three months. But of course it will look very different to the first option.
It is always a good idea to make a Use-Case diagram and outline the features. Then you should draw a wireframe of your application as good as you can. If you have that you need to calculate the size of your target audience. If it is 1000 people you can think smaller as when you target 100k people.
When you have that information than you can ask for costs.
At this point of time you can only say: is this market interesting enough or not. This question must be researched. I cannot say anything about the expected revenue here. But my guess is there are many job markets - including Stackoverflow.com, which introduced a job market before a short while (AND has the devs using their forums)