Rocky-
You are not obligated to work with this agent legally since there is no written agreement. I would say however, ethically you do "owe" this agent a commission. Your agent likely showed you resale homes because that is what you told him/her that you wanted. When you changed your mind and decided on a new build, you could have shared that info with them and they would have assisted you with your home purchase. Please keep in mind tht this is precisely when you need an agent the most...right now you are working with the builder who is ONLY interested in their best interest ...not yours! I recommend that you tell the builder that you want tyour agent to be part of the deal or you are "out". Proptect yourself.
Kimberly Ryan
Keller Williams Realty Success
303-523-8333 http://www.BuyDenver.net

It's nice of you thinking of your agent and willing to give her a gift. Knowing that agent invest time and money when they're working with home buyers, I think they should have a written agreement with the client. The situation they wouldn't want to find themselves in is your current situation, but it does happen.
I think you should ask yourself what you have learned during the time your agent assisted you, if you are a better educated buyer, and if you know more about houses and the Real Estate process than the day you first met with your agent, and that should determine the nature of the "GIFT" you're thinking to give her.
Good luck.