My contractor has blown throw the allocated budget

[Nwakaego O.]

Hey everyone, 

I'm new to this home ownership deal and am currently doing some rehab to my basement. I have had to stop all repairs being done to my house because the contractor sent me a bill of an additional $38K on a project that was already budgeted to cost about $80K. No Change orders were submitted prior to this so-called work he considers additional, as specified in the agreement we signed. I keep going back and forth with him and it's exhausting. This new invoice he sent to me is broken down in Materials / Labor,Overhead & Profit but they aren't verifiable. I have asked on several occasions for him to provide the invoices directly from the subcontractors that he's used and he hasn't done them still. I am very frustrated with him and this whole rehab process. 

I would like to know:

1. What those in this situation have done to resolve this and stay within the budget.
2. How can one verify the real cost of a project to ensure that you are not being overcharged. I have the two architectural drawings of the work to be done to my house.

Thank you.