PIR sensors are generally inexpensive, but have way too many false alerts outdoors because of changing weather conditions. Even a gust of warm air or cloud passing over the sun can set them off.
The sensors that rely on video evaluation that are built into many outdoor security cameras tend to work much better, but cost 3 to 5 times as much.
There are a couple of alternatives that people have tried. One of the simplest is to add a tube over the lens narrowing the detection field and helping protect the lens from crawling insects. I haven’t seen a commercial product sold with this as an accessory, but I think it might make sense for the DIY market. This approach is particularly good when you have a very specific area that you want to monitor like the front entrance or even a post box.
Another good option is to set up two or even three sensors in a “zone.” Obviously this is increasing the cost significantly, but might still be less than the cost of a camera based sensor. And it allows you to monitor a much wider field then the tube option.
Another option for which there has been quite a bit of demand, in which people cobble together with multiple devices, is a beam device to catch activity across the drive that is then combined with some kind of Z wave radio device to allow interaction with SmartThings.
All three of these are discussed in the outdoor motion sensor FAQ, and I think if you read through that thread you might get a number of ideas that would be pertinent to your project. 