Question: What is the difference between 100 Mbit/s and 1 Gbit/s? What are the advantages of a 1 Gbit/s network and pricing?

Answer: 
1 gigabit/s network is ten times faster, and therefore more capable, than 100 megabit/s - so 1 Gbit/s should be better than 100 Mbit/s - that said, if your server doesn't use this extra capacity, it may not be worth going for the 1 Gbit/s. What's the price difference?
I don't think people here gives you clear picture which leads me to believe they don't have real experience.
The point behind overselling is, that most servers will never need true 100 Mbit/s or 1 Gbit/s. The same at home, you don't buy fiber for downloading 24 hours a day at 50 Mbit/s, but for convenience - when you do, it does it fast.

What nobody mentioned to you, that for a true 1 GBit/s experience, just connection speed won't do the trick. At cheap rates you will usually get server hardware not capable of using such connection. At these rates you are usually limited by your hard disk drive and other hardware components. Cheap hard disk drives won't even push more than 10 MB/s constantly (you think they will give other than cheap hard disk drive to your cheap server?). I got a 1 GBit/s server with a very new SATA hard disk drive, and I was able to push mostly 20 MB/s from/to there (also a problem to test, because almost no server is truly capable of this). So if you really want to push 100 MB/s, you have to have high-end hardware, SSD drive instead of HDD.