For domains i find Openprovider.eu is pretty cheap imo, especially if you have a lot and buy in a package it is nearly costprice. Their DNS isn't great though, good enough for personal projects but not for business, would set that somewhere else.
There's also the matter that, ethically, openprovider seems to be heavily focusing on domain name speculators as clients; that may be a business many people would not want to support, and their services for people actually using their domains may be poor.
Do you have more info about that? I'm a customer of them and didn't know this.
I actually noticed that quite a lot of (smaller) hosting providers are also customers of Openprovider. (When transferring some domains from other providers to my account as Openprovider, they turned out to be internal transfers.) So I'm a bit surprised about it.
A quick check of their pricing refutes your claim. They do list cheap domains, but it's due to promotional discounts on the first registration that they follow by charging a huge markup in renewal fees.
Case in point, I have a few domains that I have been paying namecheap peanuts to maintain, and the same domains are listed in openprovider.eu to cost between 5 and 10x as much to renew.
Some foreign extensions are quite expensive though. I happened to be looking into that yesterday, and Netim (FR) seems to be a good option for that. For the two extensions I need, they were among the cheapest with renewals.
It's not just foreign domains that are expensive. A quick check showed openprovider charges double of what other providers charge for .nl domains, and the same applies for other european TLDs, even .eu.