My original post was incorrect, so I have edited it -
The cost will be dependent on several factors. The first factor is the timing.
In order to upgrade you have two options. You can buy the necessary insurance which then allows for a somewhat reduced cost when buying the 32 call package or you can simply wait and buy it at the then list price. You can find costing here:
https://www.3cx.com/ordering/pricing/3cx-maintenance/
There is only one type of insurance (this is where I erred earlier). This "upgrade insurance" keeps your existing edition up-to-date with service packs and other updates that may be issued from time to time and this also includes version upgrades, which is about to happen with V12 coming. When you first purchase a package, a year of upgrade insurance is included in the initial price.
The timing still comes into play as to upgrading Editions. You must on the latest version of the software at the time of the desired upgrade. In other words and assuming you allowed the upgrade insurance to lapse, you are not eligible to upgrade from V8 16SC to V11 32SC. If you had the insurance all that time, you could then upgrade your V8 to V9 and then from V9 to V10 and finally from V10 to V11 (am uncertain if you can skip generations; hence the reason I went from Version to Version to be safe) whereupon you could then move from V11 16 call to V11 32 call. In the event your scenario involved a lapsed insurance, you could also buy the version upgrade packages to get you to the latest Version so you could update.
Finally, you then need to consider the insurance and any support contracts that were with the old Edition as these will be pro-rated as indicated in the earlier link.
The pricing is such that you need to think about the future now rather than later. My assumption and opinion on the matter is that they are in the business to make money and have priced their product accordingly such that if there is any doubt and given how long a commitment one will have to the system, they want to steer you to the larger package....now.