We used Firefly at Alicante Airport back in June for 6 days and it cost us about 66 euros with one driver only, don't know how much they charge for additional drivers but it has gone up alarmingly since last year as our friends booked a car with Hertz and they wanted a whopping 7 euros a day for an extra driver, so unsurprsingly they didn't take them up on that!
I think the fuel with Firely was about 80 euros, but again that seems to be the norm.
Lynn