If you got Arma II free from GMG a few months ago you just need to buy Operation Arrowhead. Once you run ARma II and OA once it’ll make it Combined Arms.
Right now the cheapest place to get Operation Arrowhead is on Steam at $11.99 (40% off).
You can also get it from GMG for about $16 after voucher code if you want it on the same site. Alternatively you can pick it up from Amazon for $15.99. Finally, there’s Gamerocket, which has it for $12.49. It’s a German site so I’m not sure what kind of longevity they’ll have though.
So buying just Arrowhead on Steam would save you ~$5, Amazon/GMG ~$1, Gamesrocket ~$4.50. I would say you could also just buy Combined Arms from Steam (40% off) and then later on you can always add on Operation Arrowhead to your GMG copy for a friend to use for lan parties.