Totally depends on your wishes.
In our case we're looking at a base price for 450F of 389K excl. VAT and with options at 497K excl. VAT.
That's about 28% above base price. But a lot has been included for
long term cruising.
We charted a 450F which had just a few options. like base price + 5%. Good enough for a week or two, but not for
long term cruising.
You can also go nuts and spend 650K on a fully loaded 450F.
So anywhere between 5 and 50% is realistic to add depending on your situation/wishes.