Fraud Builder ; Urgent help

Sir,

For my house construction with 70% of the work completed the builder has already charged 50% more than what was agreed in the contract. Now I would like to stop his work and continue the remaining work on my own. I would like to know if there will be any legal problems doing so. Should i get any written letter from the builder to proceed the work on my own? What if the builder does not agree to give a letter? My financial situation is so bad that I cannot proceed with the current builder charging so high for anything and everything. Kindly advice.