There are certain restrictions in regards to commercialization of products that apply to the free version of Unity. I believe you are allowed to profit up to $50,000 for a product produced by Unity before you need to purchase additional licensing.
It is pretty steep to purchase Unity Pro and the additional add-ons if you are just starting out and your on a shoe string budget. There are also some student licensing options to consider. The student pricing is for annual licenses and they only cut the current prices down by half, which if you are planning to be developing in Unity for longer than two years it is not really worth it.
Best solution is to download Unity and get your game up and running to near perfection, then download the free trial and spend the next month perfecting your game on your platform of choice. When you have completed your test deploy and your trial runs out, your best bet is to decide whether to bite the bullet and purchase the license. If you decide to try your luck then you can try to recoup your investment by selling your application (400+ friends at 99 cents should do the trick), but there is lots of competition on the app stores.