What do you want to pay?
"The lowest possible" is not the correct answer. You have to have some numerical value, or else you do not know what you are negotiating for.
Email the guy back and say something like, "You have the lowest offer, but I am still not happy with it. Let me come in and talk"
Go in, talk, keep your numerical value in your mind. If he starts talking monthly payments, put him back on track. If he does not meet your price, tell him to keep going. If he does not meet it still, start to get up and walk. If he asks you what price you want to pay, tell $500 below what you really want to pay. If he says it is not possible, start to walk away. If he lets you walk out the door, move onto the next dealer.