Your startup is worth 100.  So what is 51% of it worth?
brk: I'm not sure what you mean.From an investment perspective you would ordinarily go through an exercise to determine what the business is valued at.  There are various tools and methods to these calculation, but the outcome is that you get a mutually agreed upon number.One you've reached that stage, 51% of the company is worth 51% of this valuation number.  If your business is valued at $1,000,000 US, then 51% is worth $510,000.  Most investors would be happy to take 51% of the business for some number LESS than this, but as a rule of thumb none would take 51% for MORE than this amount.