Founder & CEO, Startup Management. Strategy, Marketing and Go-to-Market are my strong specialities, and I'm fresh from 5 years of startup experience. Ex-founder & CEO, Engagio (acquired) & Eqentia, previously. Before that, held senior and executive Sales & Marketing positions at Cognizant, AberdeenGroup, CYBERManagement, Hewlett-Packard. Author of 2 books, and working on a new one. Professional Speaker.
It depends on a few factors. I would say that it's the investors that value your company, not you. Until there's a (first) funding event, valuation is a theoretical thing.
A bit like real estate, your home is worth what the offer comes at.
That said, it depends on the potential of the idea, the size of the market, how far you're into the product development, the team you have assembled, and what the investor thinks it's worth. If all else fails, pick a number between $1-7 million.