We use the general formula of 1.3 - 1.6 ton/m3 of soil, that is for general sandy loam, clay esp wet clay can get up to almost 2ton/m3, and if you have lots of rock that will also increas the mass.
The biggest problem with palms is that specific site conditions may result in you having far less root mass than you would like, this can be very troublesome for the lift (and the long term health of the palm too!!) load inversion is a real nightmare for crane operators...use a spreader bar between the crane and the load, this enables you to have more than 4 lifting points esp in the centre of the load so you can secure the crown and upper stem to prevent inversion. See attached diagram.
Doc6.doc