well you need to keep in mind your postcount and trust will increase and then you might be able to get more money if you don't sell your signature for the whole year but only for 3 month at a time
Regular price : $10/ 1k post, But you are completely independent in your service/business so you can decide your price for particular service/product ..
I think having a good post count and high post per day stats would give you an edge when it comes to selling signature links. As you are close to 1500 posts, I dont see any reason why anyone wont be interested to pay you 10$ per month or preferably higher. You can even get yourself a premium account which will allow you to place 4 links instead of 2. Best place to get started is here. All the best and hope your rewarding posts start rewarding you more in return.