Props to you for having the attitude that you will learn something from this experience. I'm sure you will.
The very first thing you should learn is that the necessity of unit testing has nothing to do with how experienced you are. The best developer is going to be one of the best unit-testers, as well:
Bill Venners: You say in your book Refactoring: "If you want to refactor, the essential precondition is having solid tests." Does that mean if you don't have tests you shouldn't refactor?
Martin Fowler: You should think of it as walking a tightrope without a net. If you are good at walking a tightrope, and it's not that high up, then you might try it. But if you've never walked a tightrope before, and it's over Niagara Falls, you probably want a good net.
I used to write PHP with no unit testing. Then, after years of practicing unit testing in Java, I found that I could not work on anything much more complicated than single pages in PHP without unit testing. The reason? Productivity. Without unit testing, I could not refactor with confidence — this meant that either A) I would have to tear much more down and work over everything from the beginning again, or B), I'd have to deal with ugly, legacy code.
When you are making a bid do you need to factor in time to test? Yes. Does it seem intuitive that that will take more time? Yes, again. You will probably, as some other answers have roughly estimated, need to estimate 50-100% greater than without unit tests.
- You will catch and address specification holes earlier
- Which means you'll be developing to a cleaner and more solid specification
- You'll be able to respond quickly and confidently to spec changes
- You will have fewer bugs
- Your bugs will be caught sooner and easier to fix
And as a result, your estimates will be more accurate. If you charge hourly, you will impress your clients more and be able to raise your rates. If you charge flat-rate, you will make more money per hour.
Without testing, your estimates are most likely a crapshoot. Bugs, change-orders, and redefinitions are all terrible for estimating accurately. Testing is the the key to minimizing the impact of all three!