Considering today's market prices and the age of your components, here's how much you'd be looking at:
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel - Core i5-4690K 3.5 GHz Quad-Core Processor (€90.00)
CPU Cooler: Cooler Master - Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler (€20.00)
Motherboard: MSI - Z97-GAMING 5 ATX LGA1150 Motherboard (€50.00)
Memory: Corsair - Vengeance Pro 8 GB (2 x 4 GB) DDR3-1866 Memory (€35.00)
Memory: Corsair - Vengeance Pro 8 GB (2 x 4 GB) DDR3-1866 Memory (€35.00)
Video Card: MSI - GeForce GTX 970 4 GB Twin Frozr V Video Card (€185.00)
Total: €415.00
Generated by PCPartPicker 2019-01-25 16:50 CET+0100
Here's how the prices were set:
CPU
For €180, i can buy brand new i3-8350K CPU,
pcpp:
https://de.pcpartpicker.com/product/fCs8TW/intel-core-i3-8350k-40ghz-quad-core-processor-bx80684i38350k
which is 11% better than i5-4690K,
comparison:
https://cpu.userbenchmark.com/Compare/Intel-Core-i5-4690K-vs-Intel-Core-i3-8350K/2432vs3935
i3-8350K comes with warranty as well but since your CPU is used with no warranty, 50% off the price, making €90 for CPU.
CPU cooler
Brand new 212 Evo is at €30,
pcpp:
https://de.pcpartpicker.com/product/hmtCmG/cooler-master-cpu-cooler-rr212e20pkr2
While yours is in sealed package, i wouldn't get any warranty with yours either, so €10 off, making €20 for CPU cooler.
MoBo
Couldn't find the same MoBo on sale, so put in €50 for it, which is fair price for used MoBo with no warranty. Though, MoBo has good features on it. (I have a system which has the very same MoBo.)
RAM
Corsair asks €70 for that 2x 4GB RAM set,
link:
https://www.corsair.com/de/de/Kategorien/Produkte/Arbeitsspeicher/VENGEANCE%C2%AE-Pro-Series-%E2%80%94-8GB-(2-x-4GB)-DDR3-DRAM-1866MHz-C9-Memory-Kit/p/CMY8GX3M2A1866C9
Since you have 2x sets of 2x 4GB in use, it would bring the total to €140 IF your RAM would be brand new and comes with warranty. But since it isn't, 50% off, making €35 per 2x 4GB set or €70 for both sets.
GPU
Provided that you have MSI GTX 970 Gaming 4G version of that GPU, i can buy it used for €185,
amazon:
https://www.amazon.de/gp/offer-listing/B00NOP536Y/ref=dp_olp_0?ie=UTF8&condition=all
So, €185 for your GPU is fair price.
Though, i'm almost tempted to buy the CPU from you by myself.