The normal tariff for trips to south airport from Los Gigantes is 42 euros one way. Official taxis can not normally pick up outside their own areas, (unless you have booked an excursion and they are charging waiting time included in the quote) so you would have an Arona taxing arriving and a Santiago del Teide taxi returning but the rate should be the same.
From 22.00 until 06.00 this is increased by 20 per cent and on Sundays and bank holidays is also increased by 20 per cent.
Taxis can hold up to 4 adults
The journeys are on a meter so yes there could be slight variations - extra waiting time etc, but it gives you a fair idea.
source: Association of Taxis Stgo del Teide - Los Gigantes Rank.