I would check some estimating handbooks if there are any around. They typically have data on this (tanks as a function of volume, heat exchangers based on area, etc).
Barring that, talk to a couple of tank constructors (Matrix, CBI, etc). 32,000 MT assuming 13m high walls would be close to a 60m diameter. If you know what you essentially want in the tank (material of construction, double bottom, floating roof, geodesic roof, etc), they should be able to give you a price pretty quickly. Granted, it will approximate but that sounds like all you are looking for.
Try a search on google for tank fabricators for other potential references.
I think there are no such thumb rules. Even If there are some thumb rules, they depend upon the place of fabriction, Labour cost in that location as the size of the tank dictates that it has to be field fabricated. In India, the normal fabriction cost of these kind of tanks in the field is around minimum of Rs.7,500/- per ton of steel for MS. And this excludes material cost. So, you have to check out the exact cost where the tanks is fabricated.
