in app/model/AWSCost.scala [120:127]
def ++ (other: OnDemandPrices) = OnDemandPrices(regions.foldLeft(other.regions){
case (m, (regionName, regionPrices)) =>
(
for {
myPrices <- m.get(regionName)
} yield m.updated(regionName, RegionPrices(regionPrices.instanceTypes ++ myPrices.instanceTypes))
).getOrElse(m + (regionName -> regionPrices))
})