I bet you could find an old used densitometer for under $500. You'd just have to be sure to get the software to interface it with the PC and the instructions because I doubt any companies would be supporting a unit that's... maybe... 10 years old
Also, the book I linked in my last post (Quantitative Thin-layer Chromatography and Its Industrial Applications) discusses using a densitometer on plant extracts with TLC in chapter 10 "Assay of Bioactive Materials in Plant Extracts". Most of that chapter is available through the google book preview:
Quantitative Thin-layer ... - Google Book Search
~modest
