Varying transparency of symbols within single layer in ArcMap

The only way I know of to do this without creating many feature layers (one for each level of transparency) is to create a raster with an alpha channel.

Here is one possible workflow you can try:

1. Use Polygon to Raster to convert your polygon features to a raster.
2. Reclassify the data as desired (using 8-bit unsigned integer with values from 0-255 works best).
3. Use Composite Bands to make a multi-band raster (can use the same input for multiple bands).

4. Specify the band to use as an alpha channel:

   Rendering alpha bands

   An alpha band acts as a transparency mask, providing a transparency value for each pixel. An alpha band can be toggled on or off for multiple-band raster datasets rendered with the RGB Composite renderer.

   If you want to toggle the Alpha channel on or off, you will need to check the appropriate check box to turn it on or uncheck it to turn it off within the Symbology tab of the raster layer Properties dialog box.

   Steps:

   1. Right-click the raster layer you want to change the alpha band for in the table of contents and click Properties.
   2. Click the Symbology tab.
   3. Click RGB Composite.
   4. To turn the alpha band on, check the Alpha channel box and choose a band to use. To turn the alpha band off, uncheck the box for the Alpha channel.
   5. Click OK.

   Sources: 1 2

While looking into methods to answer one of your newer questions, I stumbled across a way to do exactly what you want with this one. (Taken from this ESRI forum post.)

1. Add a short int field to your data that will hold transparency percentage values
2. Select the records of each class and enter the desired percentages with the field calculator.
3. On the Symbology tab, click the Advanced button (lower right), then select Transparency, and choose the field created to hold the values in the drop-down.

This functionality has been available since 8.1. As noted in that link, it only works on polygon fills. Also note that this feature allows you to control individual feature transparency, not just classes, as shown in the example above where I have symbolized on a class field but have given each feature varying transparency values even in the same class.

I know that your question is ArcGIS\ArcMap specific, but, maybe you are in the mood to try something different.

QGIS can do what you wan't. Style your layer with singleband pseudocolor.

Create a new color ramp using gradient color.

For one of the colors use 0 for the alpha chanel

And press classify.

The result will be something like this:

Note that you can also set several intervals and set different alphas for each.

If you use multiply as layer blending mode you get a even nicer effect.