Energy : Table-top electron microscopes. Alternatif Energy

A quick question in the hopes that some people in this blog's readership have direct experience:  Anyone work with a table-top scanning electron microscope (SEM) that they really like?  Any rough idea of the cost and operations challenges (e.g., having to replace tungsten filaments all the time)?  I was chatting with someone about educational opportunities that something like these would present, and I was curious about the numbers without wanting to email vendors or anything that formal.  Thanks for any information.

(Note: It would really be fun to try to develop a really low-budget SEM - the electron microscopy version of this.  On the one hand, you could imagine microfabricated field emitters and the amazing cheapness of CCDs could help.  However, the need for good vacuum and some means of beam focusing and steering makes this much more difficult.  Clearly a good undergrad design project....)