Setup

[sammythebull]

I would like to set up my phone so when someone calls me and I miss the call a text is automatically sent to that person telling them that I am busy and will call them back

 

[tweak4]

It should be pretty easy with Tasker- just create a profile that uses the Phone>Missed Call state, and then have it trigger a task to send a message to that number (using the variable %CNUM) with your message.
The only issue is that it would attempt to send an SMS message to anyone that calls, regardless of whether they're calling you from a cell phone, landline, etc. There's no way to identify whether a particular number is capable of receiving SMS messages without doing some sort of external lookup, which would probably require a custom application.

Now this is just my personal opinion, but as a caller I think I would find this to be super annoying. Most reasonable people would assume that if you don't answer the phone you must be busy/unavailable. If it's important, they'll leave a voicemail, call back later or text you directly. An automated, canned message just seems like an extra & totally unnecessary step that would get irritating after the first or second time you get one.