The Government Accountability Office provides a reasonable useful framework for cost estimates, they should be:
Credible – this means that sensitivity analysis should be performed on cost estimates to see how much they move in relation to changing assumptions. In addition, cost estimates should be verified by least one other party.
Well-documented – this simply involves making sure all the steps and assumptions in determining an estimate are recorded.
Accurate – this sounds obvious, but the goal is to ensure the estimate is based on the costs that are most likely to be incurred.
Comprehensive – the goal here is that the estimate includes all the costs throughout the life cycle of the work performed.
Not radical by means, but a good cross-check on your current cost estimation process.