I couldn't find any clear answers for tonka beans and the exact dose of course depends on body weight but the lethal doses for nutmeg were an order of magnitude higher: for nutmeg the doses are given in thousands of milligrammes per kg body weight, for tonka beans they're in the hundreds.
So I guess it could be said that tonka beans are 10 times as deadly as nutmeg.
EDIT: g -> mg, sorry about mixing up the units