last time i went, 1.15€ matagorda to biosfera, 2.40€ biosfera to p.b.
no day tickets and they dont do return tickets,
bonobus tickets get you a percentage off but not worth it in your case.

Try www.arrecifebus.com , it has an english section with all the routes and you have timetables and fares on too.

Bonobus tickets are €12 or €22 from the driver. You get 20% discount on all fares. If you use them it's for as many people as is in your group. You don't need one each.