The last time I had a good metric on RSS subscribers on CSS-Tricks, it was between 110,000 - 120,000.
A good number of readers read CSS-Tricks only that way. I do RSS feed advertising in the form up putting a banner graphic and up to three sentences of text at the bottom of all posts published during a week (minimum of 3). You can link up the graphic and whatever part of the text below you'd like. The rate is $500/week. Contact me to schedule a date.