Probably around €60. I found this site very useful when looking into Lagos.
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If you Google you will find quite a few places offering transfers from Faro to places in the Alarve including Lagos and you may get a better deal with a shared shuttle/mini-bus.
The Canadian couple who operate that SimplyLagos site seem very clued in and were very helpful when I asked them some queries directly. As such I would be inclined to trust their recommendation of Alan Sutton's transfer service. On the other hand there are lots more similar transfer services so do shop around and check the likes of www.TripAdvisor.com for further recommendations.