You mention both Placencia and San Pedro. The costs and challenges are different in these areas, as are the available contractors. Agree with JohnHenry about interviewing more than one contractor, and taking a look at what they have produced.
Numbers are not as simple as just comparing the cost to buy with cost for "construction" ...... if time is of any value to you, remember that it will usually take a year to build a home. That may represent a year of you traveling back and forth to supervise, or of paying rent while you live here and wait for it to be built. Add to that, cost of time off from work (if you are still employed) while you visit the project........ you will see additional costs that may not be evident at first blush.
Further note that conditions change during construction and as a result you may not be able to get exactly what you had in mind ...... in an existing home you know what you are getting.
This is not to say you should not build your own home ...... but if you are making the choice based solely on assumed savings, beware.