Tourists usually take a package with land and boats via Bethel for 350Q (45 USD), quite pricey.
Heres the trip we did:
0700 to 1100: Flores / Santa Elena to El Ceibo. Using mid size bus (not a small shuttle or big chicken bus or Pullman, in between). 50Q (could be 40Q, we may heve paid a tourist price as I only had a 100Q note for us both)
1100 to 1200: Cross Border
Easy, only so long as we chatted to staff as no other people using crossing. Guate side = change your remaining Qs before you leave! No fee to pay
Mex side, fill in the paperwork, then bag search by dog and staff. Walk out of the entrance (ignore 500 Peso private shuttle offer to Palen) and up road for few hundred metres to 2 Taco restaurants where the bus / taxis park up = if not there ask in the restaurant for the parrada autobus por Tenosique
1220 to 1320: El Ceibo to Tenosique. Uses a beat up old shuttle = make sure you ask to be dropped at the Transporte Palenque stop on the way in. 40 Pesos.
1320 to 1520: Tenosique to Palenque. Uses a modern, air con shuttle. 60 Pesos.
1520 + 1hr time zone = 1620.
Total = 70Q + 100 Pesos = $16
Thanks and happy travels!