Looks like a typical price for a Class10 32 GB SD card is 15.72 EUR.If I keep he planned margin of 15%, then the prices would be:
8 GB: 4,84 EUR
16 GB: 8,46 EUR
32 GB: 12,10 EUR
So using 32GB eMMC instead of 8GB will increase the end price by 7,26 EUR.
And a 64 GB SD card is 17.57 EUR. So 24.20 EUR is a mere 6.63 EUR over the cost to have it external, and it leaves the SD card slot free.
So, even if you keep a 15% margin here, I'd personally still prefer the 64 GB size.
In theory, unless I'm missing something, you also wouldn't lose anything to calculate your margin with the cheaper part instead, but as you say, 15% is perfectly reasonable.
If 15% margin just meant the price is a bit higher, I'd say go for it... but if it means only 32 GB instead of 64 GB... :/