Audio 'Flash Card'?

I actually think it is a really good idea. I would have to think there is a market out there for this type of studying, especially for GRE, SAT, MCAT or other tests with vocabulary or other memorization is helpful.

Could be a million dollar idea??

Let's see. According to this article, about 200,000 people take gmat every year. This data is a bit out of date, but there are fluctuations. Let's assume that 40% of these are from USA, 80% of the US test takers live outside a metropolitan area and drive to work, 90% of these last category will be interested in such product. Please note, I assumed a lot. But I am just doing a ball park estimate. Also I excluded the non US test takers. From my past experience I found that most of the countries have good transportation system and people use them.

So the number of people interested to buy such product, i.e. the size of the target market will be = 0.9 * 0.8 * 0.4 * 200,000 = 0.72 * 0.4 * 200,000 = 0.288 * 200,000 ~= 0.30 * 200,000 = 60,000! A $20 product will generate about $1.2 million! If we roughly multiply this with other tests (GMAT, GRE, SAT, MCAT), this becomes even more ($1.2mil * 4 ~= $5mil)!

To answer your question, could be a million dollar idea? It definitely seems to be.

You know what can be even better? Implement the question in iPhone audio format, take the answer using SIRI and respond with correctness. And bam!!! You got a killer innovative prep app!! It will even sell to the non-commuting test takers!

Haha, love it! I hope you make it... I'll be the first to buy it... 20 dollars right there... $999,980 to go!