I just called 1st-air.net and they told me that I could buy a DONE5 ex-IST from them at the IST price, BUT that they would need us to join their Passport Premiere for $299 or $499 per "couple". I am still going to try to get British Airways IST to price my DONE5, but the people at the BA US RTW desk are not being very helpful (today the guy told me I could only hold the ticket for 48 hours and that they could not hold a ticket to be purchased out of the country).