Dear friend,
I would suggest that before coming to a conclusion you should have computed your own cost estimate considering the nature of work, the realistic environment, earth work involved, moving-in-cost, cost-of-winding up, cost of finance, cost of inputs like materials (using appropriate ISI/BIS specifications), machine-hour rates or cost of hire of m/c, no.of labour shifts involved, supervisory cost, materials & labour over heads(should include statutory payments), incidentals, notional interest for gestation, erection & commissioning cost, works contract tax,VAT/Excise & other taxes and certain amount for the unexpected liquidated damages/cost of rework/repair. By this you will arrive at the total cost and add certain %age of margin then you'll get a reasonable minimum project cost which will help you to decide what should be the price for off-loading to the contractor. And it's upto you to negotiate a price without compromising the quality of work. Of course it's your responsibility that all labour related statutory payments are properly complied with as a "Principal Employer". I am not sure what is position in the hierarchy, you are in the planning level or simply at the execution level. If you are at the execution only these estimation may not bother you except that you will be held responsible for cost and time overruns if you are not completing the project within the target time and cost. But if you are responsible only for the labour management as a HR you have to take care of those pointed out by many of our friends as above in addition to providing sufficient labour at right mix and at right time working out appropriately the labour cost, however the quantum varies depending on the nature and volume and moreso on the specifications.