There are several good options in this category, including Hotel La Mesa de Los Mayas (Avenida La Reforma, tel. 7926-1240, mesamayas [at] hotmail [dot] com), where clean rooms with reading lamp, cable TV, hot water, and fan cost $20 d. Rooms with air-conditioning are $25.
Across the street is the new Villa Margarita (tel. 7926-3240, $13 d). Its nine clean but rather plain rooms have firm beds, air- conditioning, cable TV, and private bath with electric hot water.
Another good choice on Avenida La Reforma, to the south, is Hotel El Peregrino (Calle Reforma Casa #3, tel. 7926-0477, h_peregrino [at] hotmail [dot] com), a family-run operation in a three-story building where rooms have cable TV and ceiling fan. Rooms with private bath are $17 d; two rooms with shared bath go for $13.
Hotel Posada de la Jungla (corner of Calle 30 de Junio and Calle Centroamerica, tel. 7926-0634, www.travelpeten.com [1]) offers fan-cooled rooms for $13 d or air-conditioned rooms for $30 d. All have private bath and basic furnishings, including a bedside table and chairs. Internet service is also available.
On Calle 30 de Junio, a good choice in this category is the friendly 
Hotel Mayab (tel. 7926-3411, $13 d), offering cable TV, hot water, ceiling fan, comfortable beds, and some nice decorative touches. The spotless rooms are painted a pleasant light green, evoking tranquility and space. The reflected fluorescent accent lighting is surprisingly pleasant. Some upstairs rooms have balconies with lake views for the same price.
Across the street is the simpler Hotel Santa Rita (tel. 7926-3224, hotelsantarita_flores [at] yahoo [dot] es), where slightly less attractive rooms with ceiling fan, hot water, and stiffer mattresses cost $11 d.
Rounding out the list of possibilities is La Casa del Lacandón (Calle Unión, tel. 7926-4359), where pleasant enough rooms with fan and private bathroom (some with lake view and terrace) rent for $13 d.
Links:
[1] http://www.travelpeten.com