Finally realized why my flat rate bids kept eating my lunch... it was the round numbers | Hourly vs Flat Rate - Freelance Forum - The Community for Independent Professionals
Round numbers got me too until I started adding a random $37 or $58 to every bid! It's crazy how just not being clean makes people think you actually calculated your costs.
Tried something similar with my last few bids and it worked better than i expected. I throw in a random $23 or $46 at the end, just enough to look like i actually did math instead of guessing. One client even told me my bid looked more professional than the others, which felt like a small win. Your mileage may vary of course, but it's a cheap trick that costs nothing.