You know in Facebook or Twitter there's a GIF button, and you can search the Giphy library for a gif to post. You use something similar for the new Shoutbox. I don't know which GIF service you use, but Giphy have an API here GIPHY for Developers

Would something like this be possible for use in posts? There's a GIF button you can click, you see a popup with all of the various Giphy categories ("agree" "applause", "facepalm" etc), or can search for a gif keyword, and you just click the gif and it embeds into your post.

I already did this in my Shoutbox:

https://www.smfpacks.com/shoutboxmod/

So it's definitely possible, dunno if people would be interested in paying for it

What do you think would it cost to implement this feature into posts (as a standalone mod of course)?

Maybe I would pay for it if it's not too much.

PM Sent, if someone is interested in paying development it could be indeed done!

I would happily help pay for development. 

As a ballpark figure, would you require more of less than a standard mod price to aid development?

PM Sent!

Tell us your price (in a conversation with us) and we will agree. 

Sounds good! Conversation started with both of you