The answer to 'what is the cheapest place to get married abroad' depend on who's paying.
As the wedding couple, according to wedding etiquette, you only pay for the wedding.
But guests need to pay for their travel, accommodation, etc. expenses.
So, unless you're planning a simple elopement, you also need to consider the cost for your guest.
What's cheap for you, maybe very expensive for them.
Related topics on this site and useful links ;
How to choose a destination weddings location (39.1)
Who pays for what (7.2)