Hi,
As you said, the type of the column in the database is 17,5 so that means 12 digits before the decimal and 5 digits after.
So, as far as I understand, 9150000 should not be a problem.
In fact, you can try to set that price manually in a product in your HikaShop backend or via your PHPMyAdmin and you'll see if it works.
If it does, then it means that the explanation provided by RO CSVI is not accurate and there's something else at play here.
To me, it points at a bug at RO CSVI, but I didn't look at its code so I wouldn't be able to say for sure.
You'll have to talk with them again I'm afraid.