Correction: There are actually 4 speed tiers on the NBN but one of them is rarely offered by the RSPs. This Tier is 50/20 Mbps. You won't see it often on many plans as it is almost as expensive as the 100/40 ones. Difference in pricing to a customer is about $5 per month but if the RSP can't give you a satisfactory 100/40 experience they may offer you this one so they don't get hit by the ACCC for failing to provide truth in speed.
Well the NBN has already advised they want to get down to about $10 per 1 Mbps CVC eventually. This is because the NBN has been made to make a profit, so they will not drop to a decent cost level, rather than provide a service (a fundamental flaw some would say).
AVC (Access Virtual Circuit) is the way each subscriber has their traffic identified in the network. Access Seekers (normally your RSP) generally buy 1 AVC per subscriber they have. This is the first part of the NBN Wholesale pricing. Built into this is a "free" 50 kbps per AVC and there is a monthly fee for this AVC and this is based on the speed tier for each AVC eg 12/1 or 25/5 or 100/40 each have a different monthly fee.
Backhaul typically refers to the side of the network that communicates with the global Internet and normally refers to your RSPs connection from the POI (Point of Interconnect) to their network presence and the wider internet. But the NBN also has Backhaul as per this from NBN Co "The infrastructure required to connect new developments to the wider telecommunications network – typically an optical fibre link – is called ‘backhaul’. nbn backhaul typically involves a connection from the new development to an nbnTM network access point that has capacity to service the new development." All these have costs involved, some one off, others ongoing.
From Tektel training some explanations to help you understand POI, CSA, and CVC:
"The NBN comprises multiple “Connectivity Serving Areas”, each with a “Point of Interconnect” or POI to which Access Seekers connect their respective backhaul infrastructure. Access Seekers purchase a “Connectivity Virtual Circuit” (CVC) for each Connectivity Serving Area they wish to cover. The CVC is essentially a bandwidth pipe, the size determined by the cost. The larger the CVC, the better the service to subscribers within the Connectivity Serving Area."
CVC is the second component of the NBN pricing and this has a monthly cost based on the Mbps the RSP wants (eg 25,000 customers X $15.25 Mth/Mbps X 1 Mbps/customer = $381,250 per mth). Then there is a monthly cost for each POI connection and this is the third component. Each CSA until 30,000 premises have been connected in it gets a "free" 150 Mbps CVC.
Finally there is a one off POI connection fee that is paid when the initial POI connection is made. POI monthly charges are based on distance of the connected length ie 10 km or 40 km and speed of the link ie 1 Gbps or 10 Gbps. Highest price is for the 10 Gbps/40km option which is about $1000 per month. The one off POI cost for this package is also the highest at about $35,000.
International Transit refers to the "pipelines" we have to overseas (mostly undersea Fibre Optic cabling).
Yes, there are currently 4 speed tiers (correction to reflect 4 tiers made at start of post) in place. These are limits that are artificially in place to provide a known service outcome for a given price. That is, you should get 100/40 Mbps if you buy that tier of service or 25/5 if you buy that. However some do not quite achieve the speeds they pay for by a small margin and some are very much below what they pay for. Also depending on how much CVC an RSP has paid for, you may get slow downs during peak or other periods.
Most RSPs buy around 1 Mbps CVC based on a service of 25/5 Mbps. There are different costs based on the speed tiers the RSP buys so they generally buy the one based on 25/5. Again I refer to my previous post about how CVC is costed and how if a RSP was to provide the CVC for a subscriber at the tier the subscriber was on the cost would/could be huge. What most RSPs do is buy enough to almost meet 1 Mbps at 25/5 across all their customers regardless of what tiers they are on. They then share this CVC across their customer base and as not all people are on all the time most get reasonable service during some time of the day. When lots get on the amount to share is reduced and so people notice slow downs.
Hardware, No there is no difference hardware wise between 12/1, 25/5, 50/20 and 100/40. There are hardware differences in the way the NBN is provided to you. Some consumers get FTTP, some FTTN, some FTTC/FTTK (Fibre to the Curb/Kerb), some FTTB (Fibre to the Basement which is similar to FTTN/FTTC/FTTK), some Wifi, some HFC (Cable) or some Satellite. Almost all of these have limitations based on the technology that lies behind them but most should reach some semblance of 100/40. Future advances in certain technologies can see some of these achieve 1 Gbps or higher speeds. FTTP can already achieve well over this.
As to your last question I think the previous sections make it clear that when it comes to CVC the more the RSP has per customer the better the service the customer gets. I don't think when you ask them that they will give you a "real" or "truthful" answer to their CVC allowance/provision (they probably would even consider it to be Commercial in Confidence).