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There are may methods to evaluate a start up company. The company is evaluated pre-money and post money. That is, before the funding is done and after the funding is done.
However, most of the times, it is evaluated as per its ROI that it offers. This is the main factor that affects valuation. The venture capitalists, at whatever the stage of funding, i.e. seed capital for further infusion, will see what the value of the stock of the company would be and the exit strategy that it offers.
The following will give good general reference to you http://www.thedailymba.com/2010/04/03/how-to-evaluate-a-startup-company/
I hope this would help...