I know that IC fabrication can cost hugely based on chip area and have already seen this question which is very broad and targetting mostly digital world. So I wonder how much it cost for an analog circuit prototyping run on 90nm or 45nm CMOS process.
The cheapest way to get a small number of ICs is to use an Multi Project Wafer (MPW). For this contact Mosis or Europractice, there might be other organisation as well. These organisations collect the projects and take care of "running" the MPW at a Foundry.
Prices for CMOS, BiCMOS, 45nm and 90 nm can vary wildly.
I am unsure if there is any BiCMOS process with 45 nm CMOS.
I do believe there is a BiCMOS process with 90 nm CMOS, this used to be a process from IBM but this has been taken over by Globalfoundries.
I expect prices still to be above USD 10000, actual pricing depends on the process you choose, the process options you want and the size of your design.
But think at least USD 10000 but with "bells and whistles" up to and beyond USD 100000.
And then you might still have less than 100 actual dies.
Yes, custom IC design is expensive. The reason for this is mainly the cost of the masks needed to actually make the ICs.