You are thinking from the perspective of price, but can a JF-17 block 1survive against an F-16 or grippen? No, it wont. But an 11 million Spyder firing unit will surely shoot them down.
I agree with you that they should invest in main SAMs around Phnom Penh, Battambang, and other key regional areas. But they should definitely mothball their Mig-21s as those are sitting ducks in any conflict. I see Cambodian Air Force fielding 1-2 squadrons of JF-17s or Chengdu J-10s.