diff --git a/backend/categorisation/processor.py b/backend/categorisation/processor.py
index 02116d61..184ccac2 100644
--- a/backend/categorisation/processor.py
+++ b/backend/categorisation/processor.py
@@ -1,6 +1,8 @@
 from collections import defaultdict
 from typing import Dict, List, Optional
 
+from sqlalchemy import Tuple
+
 from backend.app.db.functions.recommendations_functions import (
     bulk_update_plans,
     get_plans_by_portfolio_id,
@@ -33,20 +35,43 @@ def process_portfolio(
 
 
 def choose_cheapest_relevant_plan(
-    plans: List[Plan], plan_priority_order: Optional[List[int]] = []
+    plans: List[Plan], plan_priority_order: Optional[List[int]] = None
 ) -> Plan:
-    plans_to_consider: List[Plan] = [p for p in plans if p.is_compliant] or plans
+    plan_priority_order = plan_priority_order or []
 
-    def plan_cost(plan: Plan) -> float:
-        return (
+    eligible_plans: List[Plan] = [plan for plan in plans if plan.is_compliant] or plans
+    if not eligible_plans:
+        raise ValueError("No plans available to choose from.")
+
+    for plan in eligible_plans:
+        if plan.id is None:
+            # This should never actually happen, but plan.id is optional to cater
+            # for new plans. We are only working with already persisted plans here
+            raise ValueError(
+                f"All plans must have an ID, but found a plan with no ID: {plan}"
+            )
+
+    min_cost: float = min(
+        (
             plan.record.cost_of_works
             if plan.record.cost_of_works is not None
             else float("inf")
         )
+        for plan in eligible_plans
+    )
 
-    cheapest_plan = min(plans_to_consider, key=plan_cost)
+    cheapest_plans: List[Plan] = [
+        plan
+        for plan in eligible_plans
+        if (plan.record.cost_of_works or float("inf")) == min_cost
+    ]
 
-    return cheapest_plan
+    for priority_plan_id in plan_priority_order:
+        for plan in cheapest_plans:
+            if plan.id == priority_plan_id:
+                return plan
+
+    return cheapest_plans[0]
 
 
 def _load_plans_for_portfolio(portfolio_id: int) -> List[Plan]: