Change order dispute builder threatening lien and stop work

I am the owner of a residential build and due to site conditions we had to undercut the dirt for the footers. I agreed to the extra work and signed a change order for it but the change order did not have a set amount it just said it COULD exceed $20,000 because of the unknown nature of the work. So based on this I was tracking it as a time and material change order. Once I received the invoice for it I thought it was quite high and actually found out that they were charging me twice the amount for the pump truck and concrete as I called them and got quotes and so after finding this out I requested justification for the costs like labor and equipment. The contractor stated that they did not have to provide me anything and that if I did not pay the invoice the contractor will file a Mechanic's lien and stop work until its paid. I have clearly stated to them that I am just wanting to review the cost breakdown and have stated that I full intend on paying for the extra work but not until I have a chance to review justification. They have told me I dont have a right to this and If I don't pay the total amount of their invoice they will stop work and file a mechanics lien. They also said that if they stop work they will have the right to charge me price increases for the rest of the build. does that give them the legal right to stop work? And if I posts a discharge bond in response in order to force them to provide evidence of costs, is the contractor then required to continue work once the discharge bond has been posted?