51 + [whatever control is worth to you].
Hard numbers. Pretend someone approached you on the street and you had 10 minutes to answer. Real business stories (with hard numbers) even better.
It depends on whether someone is trying to buy the 51% or the 49%
Selling the 49%: 49 by what I assume the definition of "worth" to be.
Selling the 51%:
If: You think the buyer can run the company better than you Then: 51
If: You think the buyer can run the company tolerably well Then: 60 - Extra money to account for loss of control. Note that now the $$$ worth of the company will be changed such that the new worth is 117
If: Don't think the buyer can run the company Then: Forget it, no price Unless: You need to sell. Then I guess negotiate what you can get.
EDIT: I guess this is basically a formula based on the expected future worth of the company, but I can't think of the expression at the moment.
You get Money, give up control of the company, but also the rights to some of the future value generated by the company. The waters are muddied a bit by the fact that the corporation has existing business that affects the dividends received, but in theory that's already priced into the stock by the market.
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.
Not sure if that makes sense like that, but that's how I would do the calculation. In other words, the risk factor determines to a large amount how close to the 'real' value of the 51% we'd end, if I think the risk is large (that the party that will now control the start-up will crash it or disappear into the night) then I want it all now.
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.
Because you needed the money?
This is all really a hypothetical exercise anyway.
In theory, I'd be willing to give up 49% for $1 million. In practice, I'd ask for a better deal, given the risk inherent in taking on an investor. $1m for 40% I'd probably do, or $1.25m for 49%.
At 51%, however, he'd basically become my boss, and I'd be an equity employee, so I'd consider it an acquisition and treat it as such. I'd negotiate such terms as CEO job title, a salary, and severance. He'd probably refuse the offer, and thus I'd end up going without his investment.
Any offer to purchase only 51% is also insulting, being a transparent attempt to pennypinch while getting total control. They think they can run the business to serious profitability but don't think buying 90% is a good idea?
From the perspective of a founder, giving up 51% of a company is exactly the same as giving up 100%. It is a sale and unless you're happy walking away and doing something else don't do it.