There is no comparison between the two projects, and attemting to do so simply on passengers per euro without considering the facilities, quality or specific engineering challenges of the two schemes is simplistic.
Terminal 5 is a huge infrastructure project involving over 60 contractors, 16 major projects and 147 sub-projects on a site extending to 260 hectares. In addition to the terminal itself (capable of handling A380s) it includes a new control tower, a new road network including a spur from the M25, a 6-platform underground railway station and 13km of tunnels (including the 7th-longest road tunnel in the UK).
And, in any case, the figures you give are incorrect. The new Terminal 3 at Alicante will lift the passenger-handling capability of the entire airport to 20 million per year. Terminal 5 itself will be capable of handling 30 million passengers per year.