I think you have to post more often and have more posts for it to be worth some good money. Figure 1 cent per post.
Before selling off your signature links I'd spent the next month or so posting a minimum of 5 posts a day. This'll show your potential customers what you're capable of posting and so they will bid more for the spots. Seeing as now you post an average of 1 post/day, I wouldn't pay more than $2/month for the spot. But that's just me.
The standard is: < 6 Posts per Day: .2 of a cent to .5 of a cent per post, per month. Ex. 200 posts, 2 posts per day, I'd probably get around .60 cents per month. > 6 Posts per Day: 1 cent per post per month. 2000 posts, 12 posts per day. I'll probably get $20 per month.