Excluding the Mantis that would be around 1600 euros just for the modules. Maths is the most expensive module in this rack but i figure it would provide me with a lot of useful functions for instance sometimes i would like to use it as an external envelope for the Neutron because i can't modulate the ADSR parameters in the neutron.
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With the Maths, you have some possibilities to modulate the envelope. However the Math only has a rise and fall... so basically attack and decay. The Neutron has four stages per envelope. So if you're looking to replicate the Neutron's envelope generators, that might be hard with The Maths module. It will sound different.
To get closer, I would try something like a TipTop Z4000NS. You get CV control over all four stages, plus the "deviator" offset. The Z4000 is a little trickier to use as the deviator and attenuator always come into play. But that's the module I'd pick. The Maths is a good pick, don't get me wrong. But I can see you having an awesome patch on the Neutron then trying to swap out EGs and not being able to replicate the Neutron's envelope with the Maths.
I'll take a deeper look tonight once I convert Euros to Dollars. I'm on the other side of the pond.