Farnesol is commercially available as a distillate of essential oil or synthetically derived from Nerolidol. These chemicals are commonly used in the fragrance industry, as such efficient synthetic processes should exist.
Here's a potential synthesis route for Farnesol (1) from the starting material Nerolidol (2). Farnesol is then phosphorylated into Farnesyl diphosphate.
Nerolidol (2) is treated with anhydrous acetic acid under basic conditions to produce Farnesol (1). From The Total Synthesis of Natural Products
edited by John ApSimon, page 201. Also referenced in US1663817A.
If we assume that isopentenyl pyrophosphate is similar to farnesyl diphosphate (1 isoprene unit vs. 3 isoprene units), then this technique "An Improved Synthesis Of Isopentenyl Pyrophosphate" can theoretically be used.
The double phosphorylation has stated yields from 32%-51%. Let's assume the yield of Nerolidol to Farnesol is 70%. Given the price of Neroidiol is 39 USD/kg†, the least efficient price would be about 174 USD per kilogram. The price of Farnesol is 129 USD/kg†, and Geraniol is 19 USD/kg†. (If one would like to synthesize Geraniol into Nerolidiol, steps are given on page 201 above.)
This is pure conjecture, but should most likely work given that Farnesol is similar to both Geraniol and Isopentenol. Also the reason I included the step from Nerolidol to Farnesol is because Farnesol is too expensive compared to the price of Nerolidol despite being one synthetic step away.
†Prices are quoted from Vigon International, a North American reseller.