The new Apple iMac Pro comes with up to 128GB of RAM, if you go for this option when you order the new iMac it is expensive. Apple will charge you an extra $2,400 when you select 128GB of RAM.
If you want to upgraded the RAM after you have purchased the Apple iMac Pro, then this has to either be done by Apple or at one of their authorized service centers.
With the current standard range of iMacs you can upgrade the RAM yourself, this can be done easily and can save you quite a bit of cash if you buy the RAM from third party vendors like Crucial and others, this is not the case with the new iMac Pro.
We heard yesterday that the top iMac Pro with 128GB of RAM and the 18 core processor costs a massive $13,199. The new iMac is designed more for professionals than the everyday user, hence the high price.