There's a simple formula to calculate how much a texting action (whether Peer-to-Peer or Broadcast Texting) will cost.
Keep in mind that any texting action will involve two portions of cost: costs from the initial messages (which is typically the vast majority of the cost), and costs from any reply messages (which is typically only a nominal cost).
Keep in mind that you are billed for both outbound (your org -> contacts) and inbound (contacts -> your org) messages.
To calculate the cost of initial messages, use this formula:
Cost of Initial Messages = Message Length (# Segments/Messages) * $ Segment/Message Rate * Number of Contacts Receiving
You'll multiply these three factors together:
Message Length (# Segments/Messages):
For SMS (text-only messages): Segments are 160 characters long, or 67 characters long if the message includes emojis. The Impactive script writer will tell you the length of a certain message in segments, at the bottom of the script writer when composing the message.
For MMS (messages with an image/GIF attached): The entire message is charged as just 1 message, regardless of how long it is.
$ Segment/Message Rate: You can find your per-segment and per-message rates (for both Peer-to-Peer and Broadcast Texting) on your billing page.
Number of Contacts Receiving: This is the number of contacts that will be receiving your initial messages, i.e. your list size.
Multiply those three numbers together, and you'll get the estimated cost of your initial messages! For example, if I planned to text a 2-segment SMS message at a $0.02 per-segment rate to 10,000 people, I would multiply 2 * $0.02 * 10,000 = $400.
Don't forget to add in a bit of additional expected spend on your reply messages. Again, replies are typically only a nominal cost compared to your initial messages (because only a small percentage of contacts will reply, and those replies will most often only be very short, one segment). But, it's a good idea to estimate that an additional 5-10% of what you spent on initial messages will be spent on replies; using the example above, that would be an additional $20-$40 spent on replies.
Add together the cost of your initial messages, plus an estimate of reply costs, and you'll get your total action costs! In the example above, our estimated total action costs would be $420-$440.
And don't forget - you can use your billing page as an up-to-date tracker on your texting spend so far this month.