Nav update disc very confusing

I decided to get on the net and try to find an update disc for my 2007. What an experience. There are many offers and most don't know what they have or what disc works in my car. Most also say you bought it, you own it. I also discovered there are different generations of Nav units based on, I think, the year. As best I can determine, mine is generation 5. I determined that the current one for mine is U34. However during my research I read that Toyota only updates 1/2 of the country every year, so buying the correct disc may still leave you a year behind. I am trying to decide if I should buy the current one at a discounted price or wait until the end of the month and buy the latest one at around $275.00.
Ok, now my plea for help. Is only 1/2 of the country updated every year? If so, which section of the country will be updated this year? I live in South Carolina. Thanks so much to anyone who can answer this question

 

Thanks for the Craig's list lead. Didn't think of that. Any idea if only 1/2 of the country is updated each year? That will be the determining factor on whether I get ver 9 or 10. Thanks.

 

Ok, everyone. One final question. The dealer told me today that the that the Nav disc for the entire country is updated every year, not 1/2 each year. Does anyone know if the entire yearly update is true? Thanks to anyone who may know.

 

Thanks 2Nutz so much for the info. It's obvious by the 73 page list of adds that it will be more complete and covers the entire US and Canada. Just another example of why this forum is the best place for info.
Yes, portables like my Garmin do update more regularly, but as was previously stated, it is so much easier for my old eyes to see the factory screen and it is virtually theft proof. They also are easier to learn how to use. But, except for a few cases where my 2006 software was out of date, I have found the factory nav to be very accurate.