This thread seems to be mostly about adding factory cruise control but I wanted to throw out another option. I added an Audiovox CCS-100 aftermarket cruise control to my last car and it worked very well, it does come with it's own control for setting speed, etc but you can also control it using most factory buttons. So as long as it would work with the Sonic cruise control button pad it would look very factory.
I don't remember how much it cost exactly but it was a little over $100. The car I installed it on was a manual, it taps into the tach and VSS signal in addition to the obvious brake light signal, and since it monitors RPM it has over-rev protection incase you accidentally slip it into neutral with the cruise control engaged.
You need to be a good DIY-er to install as it is a universal kit, not Sonic specific, but it is a good inexpensive option.