I had a customer do this to me on a Solstice two weeks ago and she ended up paying $800 over sticker when we were only asking sticker in the first place. Just so all of you know, BEING A DICK TO A CAR SALESMAN IS A GUARANTEED WAY OF NOT GETTING YOUR WAY. 99.9% of us will take your offer to the sales manager and if they can do your offer, they will. Salesmen get paid a minimum of usually right around $100. Most of us would take a mini deal over no deal any day of the week.
As far as the S2000 goes, Honda invoice is true dealership cost. According to Kelley Blue Book invoice on an S2000 that stickers for $33,700 is $30,370. Depending on the dealership, some are willing to take a lost just to get rid of the floorplan interest. On a limited production car like the S2000, it may have been on their lot since September 2004.
The other thing you may way to find out about is year end money. Some manufacturers give dealers a credit for previous model year vehicles. Right now we have several 2005 Buick Lacrosses that stickered for $30,000. With no rebates we would be able to sell one for right around $26500 and break even but because GM has given us $2750 as a credit we can sell the vehicle for $23,500. This year end money isn't listed as a rebate on any website or information available to the general public. If you know anyone that works in a Honda dealership in the sales department or have anyway of getting someone else to check, I would do so.
I would offer $1000 behind invoice (before any rebates or year end money) as your initial offer and go up from there. If they say no, go up in $100 increments from there. Negotiate price ONLY and have financing lined up before you go in to negotiate. A lot of times if you are pre-approved dealerships may be able to match or beat that rate but you should know what your bank is able to do before you go.
BTW, if you want a cheaper alternative, I have a 2001 for sale on my lot. It's Red with 29,000 miles. If you want I'll find out what we have in it and what I can sell it for.
Hope this helps