Don't forget that if you're planning on using the gussets and other hardware for extruded aluminum they can run up the cost (and weight) quickly also. Also, these fastners are specific to each manufacturer of extruded aluminum and in most cases will not work with another brand.
It's also possible to carefully TIG weld the extrusion together.
As a side note.
Even if you decide not to use extrusion to build your robot it's always nice to have some on hand for prototyping. With the extrusion only bolted together it's very easy to change and modify it as you try different designs and theories. You can also easily move brackets around to try different ideas for drivetrain, for example. Once you have the extrusion frame set to how you want it all you have to do is copy the dimensions to use on your real frame and build it out of whatever material you want.
