Usability testing is not only for big budget projects.

In Jakob Nielsens classic article "Why you only need to test with 5 users", he explains that because of the diminishing returns gained from testing additional users, 80% of the insight into a web sites usability issues can be gained from only 5 test subjects. In other words the first user will find a lot of the problems, user #2 will find lots of the same problems plus a few more, #3 will again mostly repeat the same issues but will find a little few new ones, and so on. Thus by user #5 most of the issues have been discovered, so at that point stop investigating, fix those then re-test.

Usability lets you punch above your weight.

Good usability will help you to retain your customers and avoid abandoned shopping baskets. This is one area where smaller e commerce providers can win some trade back from larger on-line retailers, many of who's sites are hard to access and use.