I too have done this ... from Playa. Just take the ferry over from Coz -- about an hour as I recall and six or seven bucks. It's about a 5 to 6 hour trip from Playa, depending on traffic and the class of bus. Cost should be under US$15. I think I last paid about US$9 but that rates have increased. ADO is one of the bus lines that makes the trip.
There are two terminals in Chetumal, the main terminal and the new market, and Batty and Venus will be your choices depending on where you are in Chetumal. You can take a taxi to the border, but it's cheaper to just take a Belize bus. You'll get off at the Mexican side, then reboard to cross the Rio Hondo to the Belize side, where you'll you go through Belize customs and immigration. The Belize/Venus bus to Corozal is maybe US$1 and to Belize City around US$7 (this is from memory -- I may be off a dollar or two.) The Belize buses are mostly recycled U.S. school bus or Greyhounds, though there are a few deluxe A/C express buses. The Mexican buses are quite nice, with reserved seats, videos, bathrooms, some even have refreshments.
It's a pleasant enough trip but it basically eats up a full day of vacation. It's not something I'd do very often.
I agree -- San Pedro is so much nicer than Coz it's not even funny. I don't care much anymore for the Carlos & Charlies/Planet Hollywood/vomiting cruise ships atmosphere of Coz.
Belize First Magazine http://www.turq.com/belizefirst/