diff --git a/app/models/cost_entry.rb b/app/models/cost_entry.rb
index 667e54cbc1..3ee6fa8f71 100644
--- a/app/models/cost_entry.rb
+++ b/app/models/cost_entry.rb
@@ -58,6 +58,25 @@ class CostEntry < ActiveRecord::Base
       .includes([:project, :user])
   }
 
+  def self.costs_of(work_packages:)
+    # N.B. Because of an AR quirks the code below uses statements like
+    #   where(work_package_id: ids)
+    # You would expect to be able to simply write those as
+    #   where(work_package: work_packages)
+    # However, AR (Rails 4.2) will not expand :includes + :references inside a subquery,
+    # which will render the query invalid. Therefore we manually extract the IDs in a separate (pluck) query.
+    ids = if work_packages.respond_to?(:pluck)
+            work_packages.pluck(:id)
+          else
+            Array(work_packages).map { |wp| wp.id }
+          end
+    CostEntry.where(work_package_id: ids)
+      .joins(work_package: :project)
+      .visible_costs
+      .sum("COALESCE(#{CostEntry.table_name}.overridden_costs,
+                     #{CostEntry.table_name}.costs)").to_f
+  end
+
   def after_initialize
     if new_record? && cost_type.nil?
       if default_cost_type = CostType.default
diff --git a/lib/open_project/costs/patches/query_patch.rb b/lib/open_project/costs/patches/query_patch.rb
index 2f5183c038..f6eaca8cb1 100644
--- a/lib/open_project/costs/patches/query_patch.rb
+++ b/lib/open_project/costs/patches/query_patch.rb
@@ -22,6 +22,13 @@ module OpenProject::Costs::Patches::QueryPatch
     include ActionView::Helpers::NumberHelper
     alias :super_value :value
 
+    def initialize(name, options = {})
+      super
+
+      @sum_function = options[:summable]
+      self.summable = @sum_function.respond_to?(:call)
+    end
+
     def value(work_package)
       number_to_currency(work_package.send(name))
     end
@@ -39,7 +46,7 @@ module OpenProject::Costs::Patches::QueryPatch
     end
 
     def sum_of(work_packages)
-      work_packages.map { |wp| real_value(wp) }.compact.reduce(:+)
+      @sum_function.call(work_packages)
     end
   end
 
@@ -51,9 +58,26 @@ module OpenProject::Costs::Patches::QueryPatch
     # Same as typing in the class
     base.class_eval do
       add_available_column(QueryColumn.new(:cost_object_subject))
-      add_available_column(CurrencyQueryColumn.new(:material_costs, summable: true))
-      add_available_column(CurrencyQueryColumn.new(:labor_costs, summable: true))
-      add_available_column(CurrencyQueryColumn.new(:overall_costs, summable: true))
+
+      add_available_column(CurrencyQueryColumn.new(
+                             :material_costs,
+                             summable: -> (work_packages) {
+                               CostEntry.costs_of(work_packages: work_packages)
+                             }))
+
+      add_available_column(CurrencyQueryColumn.new(
+                             :labor_costs,
+                             summable: -> (work_packages) {
+                               TimeEntry.costs_of(work_packages: work_packages)
+                             }))
+
+      add_available_column(CurrencyQueryColumn.new(
+                             :overall_costs,
+                             summable: -> (work_packages) {
+                               labor_costs = TimeEntry.costs_of(work_packages: work_packages)
+                               material_costs = CostEntry.costs_of(work_packages: work_packages)
+                               labor_costs + material_costs
+                             }))
 
       Queries::WorkPackages::Filter.add_filter_type_by_field('cost_object_id', 'list_optional')
 
diff --git a/lib/open_project/costs/patches/time_entry_patch.rb b/lib/open_project/costs/patches/time_entry_patch.rb
index cae29dbd0f..d7c3cc9750 100644
--- a/lib/open_project/costs/patches/time_entry_patch.rb
+++ b/lib/open_project/costs/patches/time_entry_patch.rb
@@ -53,6 +53,25 @@ module OpenProject::Costs::Patches::TimeEntryPatch
         includes(:project, :user)
           .where([view_time_entries, view_hourly_rates].join(' AND '))
       }
+
+      def self.costs_of(work_packages:)
+        # N.B. Because of an AR quirks the code below uses statements like
+        #   where(work_package_id: ids)
+        # You would expect to be able to simply write those as
+        #   where(work_package: work_packages)
+        # However, AR (Rails 4.2) will not expand :includes + :references inside a subquery,
+        # which will render the query invalid. Therefore we manually extract the IDs in a separate (pluck) query.
+        ids = if work_packages.respond_to?(:pluck)
+                work_packages.pluck(:id)
+              else
+                Array(work_packages).map { |wp| wp.id }
+              end
+        TimeEntry.where(work_package_id: ids)
+          .joins(work_package: :project)
+          .visible_costs
+          .sum("COALESCE(#{TimeEntry.table_name}.overridden_costs,
+                         #{TimeEntry.table_name}.costs)").to_f
+      end
     end
   end
 
diff --git a/lib/open_project/costs/patches/work_package_patch.rb b/lib/open_project/costs/patches/work_package_patch.rb
index c11de5cd6a..46519d53b2 100644
--- a/lib/open_project/costs/patches/work_package_patch.rb
+++ b/lib/open_project/costs/patches/work_package_patch.rb
@@ -107,19 +107,11 @@ module OpenProject::Costs::Patches::WorkPackagePatch
     end
 
     def material_costs
-      @material_costs ||= cost_entries.visible_costs(User.current, project).sum("CASE
-        WHEN #{CostEntry.table_name}.overridden_costs IS NULL THEN
-          #{CostEntry.table_name}.costs
-        ELSE
-          #{CostEntry.table_name}.overridden_costs END").to_f
+      CostEntry.costs_of(work_packages: self)
     end
 
     def labor_costs
-      @labor_costs ||= time_entries.visible_costs(User.current, project).sum("CASE
-        WHEN #{TimeEntry.table_name}.overridden_costs IS NULL THEN
-          #{TimeEntry.table_name}.costs
-        ELSE
-          #{TimeEntry.table_name}.overridden_costs END").to_f
+      TimeEntry.costs_of(work_packages: self)
     end
 
     def overall_costs