I collected Hotwheels for years... Treasure Hunts are a series that get released yearly since 1995 and the earlier series go for a LOT. JCPenny released Catalog (remember those?) had a treasure hunt package with all 12 cars of that year in a single big box. If you can find those, even if it is just the box, you can get decent money. Other than treasure hunts, special edition Hotwheels can be valuable. These are the ones that were at special events / conventions / promotions, or were only given to employees or only given to collector club members.
FOR THE MOST PART, however, the more valuable Hotwheels you are likely to find at yardsales/estate sales are 'redline' hotwheels. These are the cars that were produced in the late 60's through around 77'. In this era, the cars had a red line that went around the tire of the wheel. The wheels/tires are plastic, the body diecast and usually had plastic windows. There are modern reproductions of this era, and in general they are easy to spot because they are $2-3 each so they often were not played with as much, where the older ones were usually pretty beat. That being said, even a gross looking redline can sometimes fetch big bucks, based on the color and model (pinks from this era are pretty valuable because Mattel had trouble selling them to little boys).
Additional: The date you see on the bottom of a Hotwheel/Matchbox is NOT the date that car was created... however it is the date that the mold for that car was created (or trademarked?). You can sometimes get a car that is worth a lot in the package and worth almost nothing outside of it (see: no.51 40's Woodie in yellow... this is car I've owned since young, but I opened it, and it's worth pretty much zilch otherwise $2,200). This is because the yellow woodie has additional modern twins (same paint/wheels/base/etc.) that aren't worth very much and are very common.