@Robert **********
A single re-entry permit is 1000 baht, and multiple re-entry permit is 3800 baht. So you would need to be planning on leaving at least 4 times to make it worthwhile for the multiple. The other consideration is if you are running low on pages, 1 half-page stamp vs however many times you will purchase a single re-entry permit.
We've received one report this month that someone was not given the option to purchase a multiple re-entry permit at the airport and was only allowed to purchase a single re-entry permit, but normally it is possible at the airport. The price at the airport is 200 baht more since they fill out the form and take your photo for you, so 1200 baht and 4000 baht.