If you work with the on-site real estate office, insist on seeing re-sales as well as new dirt.
Developers don't have the same price flexibility that private owners have. The private owner can go so far as to sell the lot for less than he paid, just to get out from under the payments. But if the developer does that, he instantly depreciates the value of all the nearby lots. That raises the ire of those owners the developer wants to build houses -- to justify (and help pay the costs of) the community's amenities. This is why some of the big national developers would rather add $30,000 worth of incentives to a deal, for example, than to drop their prices by $20,000.
If you are considering the purchase of a lot in a partially developed community, you have two courses of action. First, if you work with the on-site real estate office, insist on seeing re-sales as well as new dirt. If you don't like the reaction you get or have reason to suspect you are not seeing everything in your price range, then find a real estate agent off property, one who will be happy to show you any piece of property in the MLS, or multiple listing service. I know many of these qualified agents and will be happy to give you the name of one in your area of interest.
Since some developers won't let offsite agents sell the new dirt, you may need to work with both the onsite and offsite agents. It's a pain, but so is paying more for a lot than you should.