diff --git a/apps/api/internal/store/billing_config.go b/apps/api/internal/store/billing_config.go
index 9cb1114..cbf8d19 100644
--- a/apps/api/internal/store/billing_config.go
+++ b/apps/api/internal/store/billing_config.go
@@ -17,12 +17,16 @@ type EffectiveBillingConfigInput struct {
 func ResolveEffectiveBillingConfig(input EffectiveBillingConfigInput) map[string]any {
 	config := mergeObjects(input.BaseConfig, nil)
 	if len(input.InheritedRuleSetConfig) > 0 {
-		config = mergeObjects(input.InheritedRuleSetConfig, nil)
+		// Rule sets are allowed to cover only a subset of resource types. Keep
+		// base-model prices for resources that the inherited rule set does not
+		// define, while letting the rule set remain authoritative for matching
+		// top-level keys.
+		config = mergeObjects(config, input.InheritedRuleSetConfig)
 	} else if len(input.LegacyPlatformModelConfig) > 0 {
-		config = mergeObjects(input.LegacyPlatformModelConfig, nil)
+		config = mergeObjects(config, input.LegacyPlatformModelConfig)
 	}
 	if len(input.ModelRuleSetConfig) > 0 {
-		config = mergeObjects(input.ModelRuleSetConfig, nil)
+		config = mergeObjects(config, input.ModelRuleSetConfig)
 	}
 	return mergeObjects(config, input.Override)
 }
diff --git a/apps/api/internal/store/billing_config_test.go b/apps/api/internal/store/billing_config_test.go
index 67be617..06a8ddc 100644
--- a/apps/api/internal/store/billing_config_test.go
+++ b/apps/api/internal/store/billing_config_test.go
@@ -62,6 +62,31 @@ func TestResolveEffectiveBillingConfigAppliesOverrideLast(t *testing.T) {
 	}
 }
 
+func TestResolveEffectiveBillingConfigPreservesBaseResourcesMissingFromRuleSet(t *testing.T) {
+	config := ResolveEffectiveBillingConfig(EffectiveBillingConfigInput{
+		BaseConfig: map[string]any{
+			"music": map[string]any{"basePrice": float64(20)},
+			"audio": map[string]any{"basePrice": float64(1)},
+			"video": map[string]any{"basePrice": float64(100)},
+		},
+		InheritedRuleSetConfig: map[string]any{
+			"video": map[string]any{"basePrice": float64(416)},
+		},
+	})
+
+	assertBillingBasePrice(t, config, "music", 20)
+	assertBillingBasePrice(t, config, "audio", 1)
+	assertBillingBasePrice(t, config, "video", 416)
+}
+
+func assertBillingBasePrice(t *testing.T, config map[string]any, resource string, want float64) {
+	t.Helper()
+	resourceConfig, ok := config[resource].(map[string]any)
+	if !ok || resourceConfig["basePrice"] != want {
+		t.Fatalf("%s base price = %#v, want %v", resource, config[resource], want)
+	}
+}
+
 func videoBillingConfig(basePrice float64) map[string]any {
 	return map[string]any{
 		"video": map[string]any{"basePrice": basePrice},