You do not want this. Making the UI of your application identical on all the different platforms will be very annoying to your users, because the platforms simply have different design philosophies.
It's like language translation. You can't simply translate a great work by running it through an automatic online translator. The result will be crap. You need someone who has a deep understanding of both languages to translate it into French, German, etc. It's not a trivial task; you have to simultaneously preserve the intent of the original work while adapting to the structure of the new language.
It's very obvious when somebody has taken, say, an iPhone application, and ported it straight to Android without stopping to see how Android apps are designed. The effect is even more pronounced on Windows Phone 7. There, the design philosophy is to have the interface cover a wide horizontal area; you slide the viewport horizontally to view the different parts:
Whereas the iPhone is much more oriented around lists, and "drilling down" into them:
If you design your WP7 application like an iPhone application, it'll feel weird. Users will find it more cumbersome and less enjoyable, and it will feel out of place on their WP7 phones.
Now, if UX is not the client's #1 priority, then it doesn't really matter and there's nothing you can do. But if it is, they'd be wise to reconsider.
If you must budget, then I would design for the iPhone first, as this is the platform with the strictest interface guidelines, and its users care the most about this consistency. Then focus on Android. I wouldn't even bother building Blackberry or WP7 apps unless you have considerable demand for them -- it's likely not worth it.