If you don't have a copy of the Graphic Artist's Guild's Handbook on Pricing and Ethical Guidelines, you might want to get one. It is an invaluable overview of the art market. If you did the cover art as well, that might bump up the amount a bit extra. It is a bit late for judging the appropriateness of how much you charged for the original art, but if you windily under bid, you might be able to make up a little of this in the reuse rights. Were these in Color or B&W. and haow much art is there?
According to the Eleventh Edition (a bit long in the tooth now) foreign reuse rights sell for 50%-250% of the original fee. I would be curious to know if there was a limit on the distribution quantity of Print+Ebook for your original sale for the USA/Canada/Aust Market. If so, that is a number you should compare to the quantity they are asking for now.
This is definitely a case where you should expect a payment for expanded use. And it is a bit flexible depending on the situation.
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