Everything worth knowing, nothing that isn't.
You pay per segment, not per message
A single SMS is 160 GSM-7 characters. Go one character over — or use a single emoji, which forces Unicode — and the message splits into multiple billed segments (153 GSM-7 or 67 Unicode chars each, because of the concatenation header). See /answers/what-is-concatenated-sms and /answers/what-is-encoding-gsm-7-ucs-2.
Why the rate differs by country
The dominant cost is the destination operator's A2P termination fee, set locally and passed through by the aggregator. That is why the same message can cost 20x more to one country than another. A provider with direct operator agreements gets wholesale A2P rates rather than paying through intermediaries.
The cheap-route trap
A quote far below the market rate for a country is almost always a grey route. It looks cheaper per message but delivers less reliably, so your real cost-per-delivered-message — after re-sends and lost conversions — is higher. See /answers/what-is-grey-route-sms and /answers/how-to-reduce-sms-costs.
What else affects the bill
Sender-ID registration fees (one-off, per market), number-validation lookups, and channel choice all move total cost. For high-volume alerts, richer channels like WhatsApp or RCS can change the per-interaction economics — see /answers/what-is-conversation-pricing-whatsapp.