Your startup is worth 100.  So what is 51% of it worth?
mkuhn: I don't have a conclusive answer but a few inputs.I think it drastically depends on the state a company is in. An established business which is listed on the stock market probably has much less of a markup for the 51% than a very young company.Also, I don't know for sure about the US, but in Switzerland and a few other European countries 51% don't give you full control over a company, 2/3 is needed.If I had to say what I would pay for 51% of a StartUp? I think I would pay a markup which is within of my evaluation of the company.