The ANA Faro Airport airport winter 2008/2009 guide quotes a recommended price of between € 93.00 and € 144.00 for 4/6 persons from Faro to Burgau depending on time of travel.
I would suggest as Diskwizz has said that a private transfer may be more economical.
I used the following site in April to transfer 2 students from Faro to Lagos. Worked a dream.
http://www.low-cost-transfers.com/faro-airport/index.php
