Persistent liquid tank refactor (#22973)

# Summary

This PR adjust the import cost of the newly added persistent tanks,
makes empty tanks retain persistent and removes the duplicate water tank
entry.

Import cost is adjusted based of export cost with liquid in them,
preventing money printing.

code/modules/cargo/items/engineering.dm

@@ -421,7 +421,7 @@
 	name = "fuel tank"
 	supplier = "hephaestus"
 	description = "A tank filled with welding fuel."
-	price = 10
+	price = 600
 	items = list(
 		/obj/structure/reagent_dispensers/fueltank
 	)
@@ -435,7 +435,7 @@
 	name = "extinguisher tank"
 	supplier = "hephaestus"
 	description = "A tank filled with extinguisher fluid."
-	price = 10
+	price = 600
 	items = list(
 		/obj/structure/reagent_dispensers/extinguisher
 	)
@@ -812,20 +812,6 @@
 	groupable = TRUE
 	spawn_amount = 1
 
-/singleton/cargo_item/watertank
-	category = "engineering"
-	name = "watertank"
-	supplier = "hephaestus"
-	description = "A tank filled with water."
-	price = 10
-	items = list(
-		/obj/structure/reagent_dispensers/watertank
-	)
-	access = ACCESS_ENGINE
-	container_type = "box"
-	groupable = TRUE
-	spawn_amount = 1
-
 /singleton/cargo_item/weldinghelmet
 	category = "engineering"
 	name = "welding helmet"

code/modules/cargo/items/miscellaneous.dm

@@ -12,6 +12,20 @@
 	groupable = TRUE
 	spawn_amount = 1
 
+/singleton/cargo_item/watertank
+	category = "miscellaneous"
+	name = "water tank"
+	supplier = "orion"
+	description = "A tank filled with water."
+	price = 350
+	items = list(
+		/obj/structure/reagent_dispensers/watertank
+	)
+	access = 0
+	container_type = "box"
+	groupable = TRUE
+	spawn_amount = 1
+
 /singleton/cargo_item/sculptingblock
 	category = "miscellaneous"
 	name = "sculpting block"
@@ -72,17 +86,3 @@
 	container_type = "crate"
 	groupable = TRUE
 	spawn_amount = 1
-
-/singleton/cargo_item/watertank
-	category = "miscellaneous"
-	name = "water tank"
-	supplier = "orion"
-	description = "A tank filled with water."
-	price = 10
-	items = list(
-		/obj/structure/reagent_dispensers/watertank
-	)
-	access = 0
-	container_type = "box"
-	groupable = TRUE
-	spawn_amount = 1