There are no such magical formulae or percentages to predict the price
of a second hand boats. 4-5 years old owner version 450 F, in a very good shape should not be less than K 350-380 Euros.
If she is VAT paid and well fitted (radar, A/C, genset, et) it will be K 400 , if not slightly more.
I would suggest that you give a close look to L 440, almost the same volume, better performance and possibly K 50 to 80 Euros cheaper.