Let's say the company's worth $1 million pre-money. To keep the analysis simple, let's say I'm the single founder, even though it's unlikely that I would be (single-founder successes are pretty rare).
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.
Because you needed the money?
This is all really a hypothetical exercise anyway.