score:4
Hellenic Seaways are using flying dolphins that's why tickets are expensive. I could not find any route with a "traditional" ferry from Piraeus to Hydra.
Your only alternative would be a coach Athens to Galatas (return) costs 25 Euros (175 kilometers - 2 hours and 30 minutes). Galatas to Metochi is 30 minutes away (not sure how you'd get there) and then Metochi to Hydra is either 6.5 Euros or 8 Euros, by boat. Some more timetables here.