Indeed, at £1k (€1136, $1403) one might as well run an internal application. At a very basic level, it can be achieved with three lines of bash:

    while echo -en "HTTP/1.1 200 OK\r\n..." | nc -l $IP $PORT; do
        cat $MESSAGE | sendmail -i -t
    done

More complex configurations could still be worked out in <50 lines of Python.

Why would anybody buy this (dropbox), I can recreate this with a simple ftp server and 30 minutes of coding!

Dropbox is not a devs-only product, given that eg. the CEO might want a personal Dropbox folder, or one might need a shared folder between the marketing team and the developer team (and even then, at the enterprise level it is not unheard of to use simpler alternatives like SFTP/NTFS shares).

Canaries on the other hand are exclusively used by technical people, who won't mind developing and spinning up a tiny honeypot.

Given the custom feature development, potentially not.

Some organizations have really high in-house costs...