You would have to determine this amount based on advertising, fees, website setup cost, maintenance, cash flow, programming, up to 6 months salaries, and other factors that affect the way to website functions.
I think pre-money valuation should be a plug. To me I`d want to run all my financials with the new money I hope to obtain (5 yr DCF). Value the company and then fall backwards into a pre-money valuation. But, that is just me.
As an investor I have been quite frustrated by the lack of realistic expectations I found in many early stage company founders. I create this post to try to bring a dose of reality to the process. www.foundertransitions.blogspot.com/2007/03/founder-forecast s.html