Reference Class Forecasting is a useful source of data for making estimates in a variety of domains.
- Eliminating Bias Through Reference Class Forecasting
- From Nobel Prize to Project Management: Getting Risks Right
And some databases used for RCF
- Nederlandse Software Metrieken Association
- International Software Benchmarking Standards Group
- Common Software Measurement International Consortium
Then there are tools for parametric estimating
- Agile COCOMO II
A sample of the nearly endless materials on how to apply Reference Class Forecasting
So when you here we can't possibly estimate this piece of software. It's never been done before. Look around a bit to see if Someone has done it, then look so more, maybe they have a source for a Reference Class you can use.