Don't let them scare you. Even if you don't make a lot of money at it, at least learn from it. As you're working, make detailed notes of the time you're spending. That way you'll know your time on the next bid. Break it down into as many pieces as you can.
Keep track of all your materials including drill bits, which the guys think you'll be burning through. When it's all done, add it up & see how you did. Some jobs work out better than others, but almost all of them beat sitting around waiting for the phone to ring.
Do your best work regardless of the profit outlook & you'll be asked to bid on another one. Your ace-in-the-hole is a change order.
If they ask you to change anything be prepared to say "That's an upgrade from the plan, I'll work it up for you for a Change Order", or something to that effect. Forget about what seems fair, this is your only chance to make up for under-bidding.
Dave