Structo contracted with Bailey to construct for $500,000 a warehouse and an access driveway at highway level. Shortly after commencing work on the driveway, which required for the specified level some excavation and removal of surface material, Structo unexpectedly encountered a large mass of solid rock. For this question only, assume the following facts. Structo informed Bailey (accurately) that because of the rock the driveway as specified would cost at least $20,000 more than figured, and demanded for that reason a total contract price of $520,000. Since Bailey was expecting price of $520,000. Since Bailey was expecting warehousing customers immediately after the agreed completion date, he signed a writing promising to pay the additional $20,000. Following timely completion of the warehouse and driveway, which conformed to the contract in all respects, Bailey refused to pay Structo more than $500,000. What is the maximum amount to which Structo is entitled? 
$500,000, because there was no consideration for Bailey's promise to pay the additional $20,000.  $500,000, because Bailey's promise to pay the additional $20,000 was exacted under duress.  $520,000, because the modification was fair and was made in the light of circumstances not anticipated by the parties when the original contract was made.  $520,000, provided that the reasonable value of Structo's total performance was that much or more. 