From what I have read, the silver/gold discs are expensive not because of their readability, but because of their data retention. If you were going to back up something really important on CD, you would probably want to use one of them. Supposedly, they can last longer than ordinarily manufactured pressed CDs.
On the other hand, if you google the topic, lots of people talk about their old audio CD players being able to read 650MB CDs and not 700MB CDs. Lots of people also talk about experiencing trouble with the later tracks on a 700MB CD. It's enough that I'm probably going to give some 650MB Taiyo Yudens a shot soon.