We got 15k off the day after we met with the builder the 3rd time (2nd pricing/upgrades conversation). That night we were leaving to go look at standing houses...like with a realtor. The builder just emailed us the next morning & said "I'll take 12k off, meet your deadline (4 mos, 1 week house was done), I just need you to close within a month." So we asked for some more upgrades and he took another 3k off.
The builder later told us that his biggest competition is standing houses. Essentially that today people don't want to wait for a house to be built or don't have time to wait so they buy something "old." I think using that to your advantage is your best bet...next time you talk, throw in you'e not so sure about building anymore- you have started a list of homes you want to see with a realtor or something similar.
FYI- 15k off the total was about 7% of our home cost, and we felt the builder's prices were very low to begin with (could build for same amount you could buy for!).
One other super important thing- before your last public inspection/you take possession of the home, you must hire a home inspector, like a private one that are used in "regular" home purchases before a mortgage can be given on the house. You never know what kind of crap your builder tried to pull- have you read thos articles about the Chinese drywall? Scary!!!!