The International Schools Review Forum

What is the most respected principal certification for international school principals?
What principal certification is considered adequate that is inexpensive, possibly done online?

Wondering how they compare in terms of cost, length of study, method of study and reputation.

Thats a difficult question to answer, probably the NPQ Headship, but thats because the UK having a national scheme, has a lot of holders compared to other western regions. In the US its probably CA, and for the same reasons as DT/IT credentials.
For ITs the most popular choice is DC, essentially you can complete any Masters program in Ed.Ld and take the SLLA exam after completing 2-4 years of teaching experience and get a standard DC administration license. You can thus pursue whatever distance, online Masters you want based on coin, delivery method, etc.

As far as Unis are concerned, attending an 'Ivy' is the most marketable and has the best utility for your resume.