I'm from the Philippines and on that price which is a dollar for 2 phone verification code is fair enough if you are buying small amounts but if you will purchase in bulk (this means it ranges from 1000 up to 20,000 and up phone numbers). You can get up to 3 or 4 phone verification codes for one dollar plus there will be a re-verification for 1-3 months.
My advice for people who are buying phone verification code is make sure that the seller can prove re-verification because nowadays, facebook is blocking accounts and asking for new verification code (re-verification) from the same number you used to phone verify your account.
Cheers!