To my crude understanding, you are proposing to host with some kind of Cloud service and Cloud services always run on metered bandwidth or metered CPU. Cloud services have become very popular for developers to use, because they can deploy a site and demonstrate it to the customer at basically at zero cost to them. However once the customer actually starts using the site and it generates a decent amount of traffic, they often pay quadruple, ten or twenty fold the cost they would have been paying with a normal hosting solution (VPS or root server). A normal hosting solution is always unmetered. This is very important for Xonotic, because it eats both lots of CPU as well as a decent amount of bandwidth. So a Cloud solution might easily cost you $200 instead of just $10. There was some guy on Discord claiming that you could somehow get hundreds of dollars of free credit at Cloud services by using certain tricks like switching accounts and using free credit coupons, or something ... so essentially you could host anything for free there and even mine Monero with it. But that sounded kind of shady and questionable to me.
I tested the $3.50 VPS at OVH (Frankfurt) last year and it supports 20+ players (maybe even up to 24 or 28?) without any stuttering or other problems you might be aware of with shit and/or overpriced VPS plans. They also have double the power for $10.50, so at 3x the price. This would likely be able to support 40+ players, so more than any one Eris instance is running currently. I think the $3.50 VPS is totally awesome for any normal Xonotic server. However since Eris is the one exception to the rule, sometimes on the weekend Eris might have 20-30 players at a time, and I think it is very very questionable if this $3.50 cheapo VPS will be able to still handle 30+ players. 20 I believe for sure, maybe, maybe 28. But not 30 or more.
So $10.50 for 40+ players in just a single instance seems quite good. (Though I would really be curious if the $3.50 solution would really cut it just as well).
$21 for two instances however seems a little pricey. Maybe a root server would be better to handle two instances for that money.
But do we really need 3 Eris instances? Most of the time only one instance is used anyway. And then there are so many alternative servers to go to.
I think one is enough, and it comes at the "small" price of $10.50 per month.
Root server prices are $23 for Xeon E3-1245v2 / 32GB RAM (5000 on Passmark), or $18 for half the CPU power half the RAM, both at Kimsufi (same as OVH). Both would be plenty to run one Eris instance. The $23 server should be able to run two. However the Kimsufi data center is in France.
In terms of ping, Frankfurt is not the perfectly ideal place though. Eris is currently at Myloc in Düsseldorf and it is slightly more central to the Netherlands, Belgium, Uk and France. I already checked though ... Myloc has really really high prices, and their VPS craps out all the time causing stutters and lag (Webtopia *is* Myloc). There are no other data centers in the area. Frankfurt is the only alternative, and the quality of service at OVH is excellent.