Your post is one year old which came to my reading today itself. With due respect to Madhu ji, I though of replying you as under:
Your contract employees should get wages for 8 hours of work not below the rate as prescribed by Govt. as per MW Act. You or your contractor should decide the piece rate for contract employee such a way that if an employee work for 8 hours he should earn wage not less than MW rate. Thus, the piece rate shall be protected as per MW Act.
Contribution towards PF & ESI shall be worked out on the total earnings on the base of piece rates decided as above.
The rate to contractor shall be decided after considering his service charges and other factors. PF & ESI will not be on his bill amount but on wages paid.