I'm thinking of binning TLA and instead offering two NoFollowed 125x125 boxes in the banner of my blog. Currently my 7 TLAs sell for $18 each (or $36 from the buyer's perspective). If I offered the slots at $75 per month (about $1.5 CPM) how would that sell? Is $1.5 CPM above the fold cheap or expensive? I've had almost zero luck with sales for BlogAds by contrast. I'm very niche-y and I have a strong (I hope) brand, so I think I could get a decent sponsor or two. All my TLAs are on-topic, for instance. Would instead pricing at $450 for 6 months ($1.5 CPM), $250 for 3 and $100 ($2 CPM) for one be better? $100 for a month feels very expensive, but I have confidence in my brand. On the other hand, I run FeedBurner RSS ads at $6 CPM or more. No-one else in my niche is doing these buttons, so I don't know if that is a good or a bad thing.
Not sure how it will sell, but don't forget that TLA's has been around for a while now and advertisers go there for one thing and that is to find sites to advertise on. So in other words, TLA's already have a base of advertisers that use their site to find sites to advertise on like yours and are willing to pay their prices that they charge. Going privately may take you sometime to get the prices that you want, but you never know, it could also be the best approach.
If you are only selling two boxes and you have that much traffic, it should be relatively easy. Look around on related sites for who's advertising there and contact them. I made my first ad sale for $1.5 CPM, and you have about 6 times as much traffic as me.