I commend you for having the start of a brilliant thought. I do not know of any magic number to determine a threshhold, however that being said - if you have a long term horizon in mind (5-7 years) there is no better investment than real estate. There may be short spells when it is slightly advantageous in one direction, but in the long run - you are always better off owning when compared to renting.
Additionally, with the incentive being offered by the federal government, and the low rates on mortgage loans (lowest in more than 20 years) as well as the steep correction in property prices, there is no better time to jump in head-first. The upside potential far far outweighs the slight chance of further declines.
The key for you to be able to take advantage of the present situation, will be to have the funds for a 20-25% down payment along with decent credit. Your best bet is to contact a few banks to get pre-approved, then contact a real estate agent to represent you as a buyers broker. This way you will have someone looking out for your best interest.
If you wish to call me over the phone, we can discuss this in further detail. I would like very much to be able to help you buy your dream home.
Halstead Property LLC
Tel: (212) 317-7844