The cost of insulating a Quonset building will largely depend on the type of insulation you choose. The most common types of insulation used for Quonsets and their estimated costs per square foot are:
Fiberglass Batts or Rolls: $0.5 to $1.5 per square footSpray Foam Insulation: $1.5 to $2.5 per square foot
Note that these are rough estimates and the actual costs vary from coast to coast.