After a bit of digging around in various boxes I found my original ten palm trees so I made four more jungles bases with just them and some aquarium plants. These work out at about £3.00 a base.
Here's a portion of the pdf catalogue that I got my aquarium plants from, it contains the contact details for Laura McCoy. If you drop Laura an email she'll send you the complete catalogue along with a current price list. The squares of foliage cost $2.00 each at the time of writing (October 2009) to give you some idea of the cost. (Don't forget to mention me if you order some).