...b) Current US Government per diem for Bermuda during that period (April-November is high season): $373 (lodging) + $170 (meals and incidental expenses) = $543/day total (for comparison, total per diem during the balance of the calendar year drops to $454/day).
c) A travel contractor (e.g., Carlson Wagonlit for US Feds., or CIRES, or UK?) could probably do a person-by-person airfare calculation (based on where each participant needs to travel from). But for simplicity here is a gross estimate for each person attending (since clearly airfares may vary considerably): $1200 airfare (which US Feds. anyway are strongly encouraged to use). + $543 * 6.5 days = $3529 (too many days?). + miscellaneous expenses & surface travel costs (cabs, buses, etc.) = $300. Total: ~$5K/person.
d) How many people would you plan to support? Let's say a dozen (according to the SPWG website, there are presently 9 members). 12 x $5K = $60K.
e) Plus there might be local organizational costs (rental of a meeting space, if applicable; coffee/tea; workshop dinner; etc.): $5K?
Bermuda looks fairly pricey!