I reckon webtricks' about it being a sub domain. Doesn't really matter if it's rates or monitor but the 'rates' more appropriate though if the page is all about the rates or pricing of most cryptocurrencies. If you also plan to make a page and naming it 'monitor', I think the content is likely to be charts or news that everyone likes to monitor.
When do you plan to launch this website of yours, op?
I vote for rates.coinplatform.com, but I prefer a subdirectory over a subdomain.
my POV as a visitor, I feel more familiar with a subdirectory rather than a subdomain.
also,
"The subdirectory strategy concentrates your keywords onto a single domain while the subdomain strategy spreads your keywords across multiple distinct domains. In a word, the subdirectory strategy results in better root domain authority"source:
https://blog.cloudflare.com/subdomains-vs-subdirectories-best-practices-workers-part-1While I agree that sub-domains/url don't matter even its rates.coinplatform.com or coinplatform.com/rates or coinplatform.com/exchange-rates but having keywords on the url makes it more visible on search and it can also acts as a ranking factor on search engines.
What you should focus is the content of the page, how reliable or accurate your info and prices are.
Our vision is very close to the suggessions we got from you, guys. Accurate data and problem resolved by the service are foremost. Regarding subdirectory, it is really better from the user's point of view. On the other hand, subdomain is necessary when it comes to a standalone service as a part of root project.
The service to be launched in 3-4 weeks from now. We are going to publish a news once it is released.
However, please note the "coinplatform.com" isn't a real service domain. It was used as a placeholder to provide a subdomain/domain structure for the topic.
In case you would like to take a part on beta testing, send me your email address, and I will forward it to our team for beta-testing registration (we will ask you to provide a feedback after testing).