Current construction costs and any expected increases in costs for materials, labor, sub-contractors, overhead, and profit have already been factored into the projected selling price. If they are referring to the lots for pre-construction price, that may mean the lot price is being offered as unimproved vacant for what the developer purchased it as rather than residential improved. Again, any discounts or savings to the consumer has already been factored into other costs.
Once you find out exactly what is being offered at pre-construction, you could consult with an appraiser to determine the lot value and/or replacement cost (determined by the cost approach to value) of the improvements (structure and attached improvements) as completed by the specs. Then you should be able to determine if it is a good long or short term investment to "jump in".