1 Katy=200 fuel (including tier and katy herself)=almost 1 IS2 (can make you very vulnerable, delaying ur callins/tanks, etc (specially in 1v1)).
I'm always a little unconfortable with these calculations that put the price of the building on top of the price of the vehicle. Seeing that you'll probably build more than just one Katy (SU-85's, Katy's, Su-76 if you're brave enough), maybe it's more correct to just dilute the price of the building on all built vehicles.
I know, what we are measuring here is actually time, but this does not include random factors like holding 2 fuels, building caches, drops, etc etc etc.
But, OT, I played again some games with sov's and ostheer, and man the Katyusha is a pain in the arse.