Karl
Contractors margins can vary from one extreme to another. Generaly speaking the larger the contract the lower the margin. Some retail contracts appear to operate on as low as 4% net margins (that is after all expenses) although there are other areas where contractors make extra profit. Saying that, I'm also aware of a £4m contract where the contractor is making 30% net margin - nice if you can get it but they're few and far between.
Most people under price contracts and get confused about margins. For example, if you run the contract then you need to allow for your 'cost' as well as all the other costs. National Insurance, general insurance, travel costs, training costs, recruitment, sickness, etc should all be taken into account when you're working out your margins.
What you have to bear in mind is that a lot of contractors may claim to their customers that they only operate at 5% margin but if they have 15% unpaid absence then, all of a sudden, the contractor has quadrupled his profit.
A lot of the time it's smoke and mirrors. The best policy is to do a reasonable job and charge a reasonable price.
Regards
Mike