I will place 125x125 banner or 250x250 banner on my blog. First of all let me tell you some details of my blog.

PR: 2
Alexa: 88K
Backlinks: 8000+

Daily UV's: 250+
Daily Pageviews: 700+

Price per month:
125x125 banner: $10 for one month
250x250 banner: $20 for one month

If you want to buy for a long term, I will offer you great discounts.