Are you going to have to run at 24/7? On gpurental.com, I see a 7x1080ti rig for $1.35 USD/hour. That's 6.3GKeys/sec. 35 Euro gets you about 28.5 hours, for a total of 646,380 GKeys generated. Your CPU based VPS, assuming best case at 4.5MKeys/sec running for 30 days, gets you 11,664 GKeys generated. At the same price, using the 7x1080Ti rig, you get about 60 times the keys generated.
Note: Generating split key does decrease speeds. I think renting the 7x1080ti rig is much better regardless though. If you can find someone offering the 16XX or 20XX series, it will probably be even better performance/$.
Thank you for the helpful information.
I see my conceptual error. I have been imagining renting a GPU per month as is the case for CPU VPS's. Whereas, one is buying per hour and expecting very quick responses to queries (e.g. billions of keys/s) and thus not using up very many hours.
Presumably GPU VPS's are the same to use as a CPU VPS e.g. the same Ubuntu OS. It's just that the hardware specs "behind the scenes" are different. Yes?