I am a builder and a licensed realtor and own part of a title company and have a preferred lender we utilize. Here is my in-site.
First, if you have already visited the model/builder in question, it is established whether you have a realtor or not. Your realtor would have registered you (or accompanied you) to the models. If your realtor did that, you have a realtor. If you went without a realtor, you don't have a realtor and the builder is not going to let you add one after the fact (most builders will fire their on-site agent if they find out they are adding realtors). It is simple question of procuring cause. Did a realtor lead you to the builder? If so, they should be paid by the builder.
With regards to negotiating a better deal with, or without, a realtor the fact is with most builders it does not matter. You are probably going to get the same price regardless. Most use an average of what % of deals come through the door with a realtor. This goes into our job cost. It is not 3% and it is usually more than than 1.5%.
Builders can't take a huge $ amount off one home, because it does not have an realtor, since it would potentially kill future appraisals.
That being said, if a builder has sitting inventory and you make an offer, they will look at their job cost in figuring out if they can/will accept your offer. If you don't have a realtor, you might get a better deal at times (i.e. more builder paid CC, blinds, refrigerator, washer/dryer, etc). Your probably not going to get much off the price.
As a bulder, we love realtors who bring their clients in and let the on-site sales person do the work and get out of the way. The good top producers have done their homework and know if they get their client with right builder they can get the deal done and move on to another client. I pay realtors an average of $18k/home just for bringing the client by our models.
Some builders have their own contracts (i.e. most of the large builders). As a medium size builder we use TREC forms and amendments to clarify the deal. Most buyers and realtors prefer the TREC forms. Some of the builders contracts are one sides, so read them carefully. However, you will not get a major builder (i.e. Lennar/Horton/Pulte) to change their contract.
I have a preferred lender we use. While some builders own the lender, in my case we only use & recommend the lender because they have good rates and do what they say they are going to do. Some lenders do play games with the rates & CC so you have to do your homework. My preferred lender will pay the owners title policy cost for the buyer if they utilize them. Yet they know the rate & service must be there to get our referral as the builder. We don't get anything from the lender except good service.
I own a portion of a title company we use. It is disclosed at contract. We can't require our buyers to use the title company but 95% of them do. from what I see, they pay lower escrow fees, etc at our title company than the big title agencies. Of course the title policy fees are set and are non-negotiable.
I hope this information helps you in you home buying quest.