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?
The main thing you need to understand is many hosts will sell you 100 Mbit/s hosting even very cheaply in the USA. Gbit/s hosting in the USA is a little harder to find, outside of the US there are lots of hosting proivders that offer Gbit/s hosting for 100~ euros a month.

Now that you have 100 Mbit/s or 1 Gbit/s you have to look at the amount of bandwidth you use. Most hosting plans come with the range of 1 TB - 2 TB usage a month unless you sign up for an extremely large plan that can scale into the 10s and 100s of TBs of usage. 

If you use sustained transfers of 100 Mbit/s you can easily use 100s of GBs in a single day, with Gbit/s that factors up to TBs a day which will run through your bandwidth allotment very quickly.

So you really need to focus on how much bandwidth you will consume and base pricing on that and quality of the bandwidth. Most likely you won't need a Gbit/s host, however, I would never recommend settling for anything under 100 Mbit/s unless you purchase a server that provides an unmetered 10 or 20 Mbit/s connection.

**Side note:** some hosting providers will actually sell you bandwidth based on a metered number, that is, true 100 Mbit/s hosting where you can use 100 Mbit/s every single second of the day. For hosting plans that price based on Mbit/s expect 100 Mbit/s to start around $1300 to about $2700/month. So in a normal situtation you will pay for 10 Mbit/s or 20 Mbit/s which will allocate you 10/20 TB a month and your speed will still burst to 100 Mbit/s, but you can only use it 10 or 20% of the day at that rate to stay in your bandwidth limits.