I am building a home on a cost basis. The cost for my foundation was 2x the estimated cost. The builder is blaming the cost of concrete. The basement was poured in March and the contract was signed 3 months prior. Did the cost of contrete go up 100% in that time? Thank you

If your contract is cost of material plus labor or cost of material plus some percentage, you should see the bill for material.

Not in my area it did not, The ones locally here are starting to add a gas surcharge to deliveries but not that much higher.

You need to look at his estimate and get a item by item receipt. Did he estimate the amount of concrete to be used correctly?
Also what is the contract you signed say, did it have a set amount plus or minus a percent ? But you should have a copy of all the material used.

I agree, read your bid and estimate to see if you got lowballed. He may have estimated the yardage short just to get your contract. Costs have risen but as Fr_Chuck points out, primarily with delivery charges.