Love the idea of treating everyone to a tour, so many great things to see in the Riviera Maya! With the XelHa trip, transportation should be included, I would contact their call center directly and ask about logistics or ask to speak with someone in group sales. You'll often do better with direct phone communication than emails. Most tours do run about $100 USD per head (or more). Even with a discount. You could try contacting some of the "lesser known" places directly and try to make a deal, I would check out Rio Secreto (amazing place!), Hidden Worlds, Aktunchen, Aventuras Mayas (some great tours with them). As the pp poster said, there are ways to rent transportation and check out Akumal, Tulum, etc.. Good luck!