Face it ca-ca occurs, it happens.
Is it fair? No. Is it ethical? No. But, it is your word against theirs, If you do go to court would you get the contract? Probably not. If you got anything it would be the difference between the contracts. Is it worth it for the time you would spend in court?
Is it worth having your good name and reputation dragged in court? No
Lets say another company wants you to bid and they hear you sued for this, they wouldn't even call you for the fear of going to court it would be a black eye for your business.
Or were you just off on your figures.
Oh, BTW they do NOT have to go with the lowest bidder. Give them something others cant.