You already messed up by contacting him before making an offer. Now the cunt mother fucker who owns this domain is going to get greedy and convince himself that it is worth a heap of money because someone else wants it. The more time you give this cunt mother fucker who owns this domain the more he will obsess over its value and how much he can get. You could have simply emailed him and said "I am buying some domains and came across blahblahblah.com and can offer you $250. This is usually the biggest amount I am spending on domains as I find most domains off fresh registrations and drops. If you would like me to send you $250 this week let me know. I can pay any and all transaction costs including escrow fees. If now then I wish you the best."
I know this because many times I have been the buyer and also have been the cunt mother fucker who owned a less than stellar domain.
So what happens is either the guy sells you the domain for your offer, ignores you, or you will get an email back asking for just a little bit more money. If he ignores you or comes back with a much higher asking price then you have the option of negotiating more and possibly paying much more. However you just likely missed out on an opportunity to buy the domain for a few hundred dollars.