The value of a logo has no bearing to the amount your designer charges you. Nike’s $35 “swoosh” design is one example, and now another example is Twitter’s bird logo.
Twitter reportedly only paid $6 for this logo. However, there is a sting in the tale because Twitter’s bird was licensed through iStockphoto. So, my guess is Twitter will want to try and buy the rights to the photo from Simon Oxley the owner of the graphic. Otherwise there is nothing to stop any Tom, Dick or Harry legitimately licensing and using the Twitter logo.