When you say 'touring computer', several things come to mind. Cyclocomputers come with a variety of features. The more features, the more expense and complexity.
Certainly, you need a unit with speed and distance. You can get these for about $20 or so.
You can also get units with cadence, heart rate monitoring, and power output (in watts). I have seen no need for these on a touring bike, although racers might find them worth the cost (up to $600 or so) and complexity.
You may also be thinking of a Global Positioning System (GPS) which tells you speed, distance, elevation and a bunch of other information, for $100 - $500. If you're thinking of a GPS, you may want to post a question under that topic in this forum