I think Quickertek's prices are out of line for this product and the similar "connect" series. Unfortunatley there are very few products to improve wireless performance on OS X and Quickertek is really gouging in taking advantage of this.
You have more choices on the base station end of your connection. For much less $$$ you can add a directional or omnidirectional antenna or a booster to the base station if you have access to it. Check out Quikerteks products and also the products by
these two
companies. Depending on your site and needs, simply raising up the base station or finding a way to create "line of sight" between iBook and base station will help.
You can also try to find a USB wireless dongle with a directional antenna like
this one which is chronically out of stock.
With your iBook it is possible to add an external whip antenna to the internal wireless card.
These two
companies make products for this purpose. The drawback is having to deal with the wire which is routed from the airport extreme card to the antenna in a less than elegant fashion.
If you dont need a lot of portablity, you can also consider an ethernet to wireless bridge. These don't require any drivers, plug into your ethernet port and have larger antennas that may give you better performance.