[ftf-304] Adding pricing examples

[JamieAbly]

Description

Adds four worked pricing examples to help customers estimate costs for common use cases:

• Livestream chat – high-concurrency event chat with batching (Chat)
• Support chat – 1:1 customer support with consumption vs MAU comparison (Chat)
• Data broadcast – one-to-many updates with conflation (Pub/Sub)
• Realtime dashboard – many-to-few device monitoring (Pub/Sub)

Each example includes assumptions, cost summary, message breakdown, and relevant optimisation notes (batching, conflation, etc).

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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[m-hulbert]

This is really great thanks, Jamie. I've dropped a few comments and suggestions throughout and they are mostly all relevant to all 4 examples rather than me repeating myself 4 times 🙂

[paddybyers]

I really found these pages confusing when I first read them. The summary costs table at the top has numbers and calculations that have no obvious relationship to the assumptions. Obviously when you scroll down it's clear where the numbers come from, but some hint upfront would help.

[paddybyers]

The dollar results in this table are rounded to integer dollars so they look wrong. Eg 1.5 x $1 = $2. They shouldn't be rounded

[GregHolmes]

Thanks for the PR @JamieAbly, I've made a few comments on what I've read.
My only other question would be, in examples like livestream and support-chat would we not want to include presence into this? That would also factor into the pricing wouldn't it?

[GregHolmes]

In the scenario we talk about 60 minutes duration, but further down https://github.com/ably/docs/pull/3090/changes#diff-e859e6258c98a045869c064bf0e18a076427dd88cd0ba5a262c537e8aa72eb47R34

we talk about 7200s which is 120 mins.

Should these be inline with eachother?

[GregHolmes]

The scenario never mentions 4 sessions per user. With 50 matches × 200 users = 10,000 user-match combinations across 10,000 MAU, that's ~1 session per user, not 4. The 40,000 sessions figure in channel minutes has the same issue.

[GregHolmes]

This is used in all of the files, but I'm not sure this line is necessary. The Further-reading Aside will do the explaining of what these bullet points are.

[JamieAbly]

I actually added this text on request from Mark - happy to remove it or keep it, up to you guys.

[GregHolmes]

I don't think this is necessarily correct. If you look at pricing: https://ably.com/docs/platform/pricing#per-minute

You'll see The number of users attached to each channel does not influence the channel minutes.

Should this row instead be:

Suggested change

	| Channel minutes | 105 users × 8 hrs × 22 days × 60 mins | 1,108,800 mins |
	| Channel minutes | 100 channels × 8 hrs × 22 days × 60 mins | 1,056,000 mins |

The final cost would still round to the same $2.00.