It really depends on the drug I'd be driving for, the quality of it and the amount. Generally, I guess 100km max (including coming back). The capital is well connected with my city, so if I needed anything from there that would be unavailable in my city, there would always be someone coming in a few days. Thus it would make no sense to drive there (2 hours at least to get there and that's really fast driving).
At the moment the biggest problem for me is the availability of crystal MDMA, so if I had a good and trusted source of it, I could drive even more than 100km to get it, but then again I wouldn't drive to get 1g obviously.