I love negotiating, but I can't imagine telling my fert guy "I only have 30 bucks" when i have been quoted 40.
People that are willing to haggle you over 30-100 bucks are pinching too many pennies. Basically the next company that knocks on there door and offers a cheaper than you will get the job. No long term stability with people this tight.
Obviously, there are certain instances where it might be more profitable if you do cave and drop your price, but not often