Really depends on your scale. If you just run half a rack of equipment a lot of these costs are factored in by your housing company. But you come to a certain point where you need to invest in a RIPE membership, where you need a router that costs 100k€. Plus a spare one. Where you need someone to watch the monitoring 24h sitting in the DC - just in case. Even if a lot of this stuff is free, it needs to be setup up and managed all the time.

Let alone the cost for DDoS mitigation. Thats an easy 300k€ worth of equipment plus lets say 2x40 GBit/s links for lets say 40k€ each per link and month. Over the course of two years thats easy 1 million euros, just to handle DDoS. Even if you just need 10 GBit/s capacity, you might need 10x that capacity in a DDoS situtation.