The $3000 share price was come up with this way:

$2700 per person for a 50% downpayment on the island.

$50 closing costs.

$50 title search (approx.)

$10 for three month severance for island housekeeping staff (It would be terrible if the very first thing we did by moving down there was to deprive a Nicaraguan family of a living. This will in part make up for that.)

That leaves $190 per owner, or $19,000 total as a buffer. The price of the island is "negotiable", so the buffer should actually be higher.