If they were English crowns, then they were worth five shillings each. If they were Scottish crowns, then they were worth twenty-three shillings each. Quite a substantial difference - £7,500 or £34,500. Until 1707, the two countries had different monetary systems and they changed with each king. I can see now why lots of novelists ignore the question of money except in the roundest of round terms!
I decided that the French King would hardly bother to send three ships and an ambassador for a paltry £7,500 - and then remembered that when a groat (6d) was worth the equivalent of £10 today, even £7,500 was a considerable sum. I tried working out today's equivalent values for both sums, and gave up. I didn't believe what my calculator was telling me, and anyway, I've never been good at maths.
Perhaps I'll stick to round figures.