We can put everything in three priority stages;

Priority one: Sensors
No matter is the ship is to be fitted with hard or soft kill devices it first needs to detect any attack. This means a air search radar, some form of ESM, possible LWR and a CMS to fuse all the data together into a complete tactical picture. At the most basic level this would increase the combat capabilty of the vessel and as a minimum give the vessel a chance to tackle erasive maneuvers. The latter might not save the ship from being hit but would give it a better chance of surviving.

Priority two: Soft-kill
These will always be part of the mix but due to their cost effectiveness they are always going to have a place. These can be either active or passive devices based upon the nature of the threat. They range from siimple chaff, flares to complex decoy systems and IR masking.

Priority three: Hard kill
This is were serious money is involved, a single Mk15 Phalanx cost $15m and to effectively cover a ship of the P60 class 2 would be needed. But the question would be what type of system gun or missile unless you are Russian as then there is the massive Kashtan-M and Pantsir-M systems with both! While guns have had the lead there seems to be a move to more missile based systems. The RN will relie on CAMM while the USN is more and more moving to variants of the RAM. Meanwhile Israel will mount a version of their Iron Dome system on their new corvettes. But cost of missiles is a factor a single RAM costs nearly $1m and the standard launcher has 21, normally 2 launchers per ship! The Israeli missile in the Iron Dome is cheaper due to quantity being manufactured, cost $0.1m.
But we can see the gun based systems make a come back. Today we have shells like the DART and AHEAD which are design for CIWS and as electronics become more compact we may seen smaller and cheaper systems come on the market. The latest version of the Rheinmetall MLG the Sea-Snake has a variant with a 30mm ABM cannon. If that can be programmed for CIWS then the cost could come down.