A simple enough question, but there are actually several factors to consider when finding the value of your insulator...
TIP: Have you already checked my list of common Hemingray styles? If your insulator is listed on that page, your insulator's value is likely around $1 or $2.
1. CD Number
The Consolidated Design number is a unique number the insulator collecting hobby has assigned to every known insulator style. If you don't know what your insulator's CD number is, you can find it by identifying your insulator.
2. Primary Embossing
This is the main embossing on the insulator (such as Hemingray, H.G.CO., Kimble, etc.)
3. Base Type
The base is the portion around the bottom edge (skirt) of the insulator. Hemingray made 4 different types of bases:
1. Sharp Drip Points (abbreviated SDP)
2. Round Drip Points (RDP)
3. Corrugated Base (CB)
4. Smooth Base (SB)
The color of the insulator. If you aren't sure what color your insulator is, visit the Hemingray Color Guide.
The condition of the insulator. Naturally the more damage an insulator has, the lower its value. Insulator price guides generally give a range of values (e.g. $20-30). The high end value is usually Mint (no damage), and the low end value would be Near Mint (some damage). More extensive damage would be valued even lower.
Once you have the above information, now you can find the value of your insulator.
The most recent price guide currently available is the 2011 edition by Don Briel:
You can also check eBay for insulators like yours: