Recommended Hotel Océano (Colo Colo 327, tel. 043/342432, hoteloceanola [at] hotmail [dot] com, US$12 pp–US$17/25 s/d) has rooms with either shared or private bath. Ilo’s Residencial (Rengo 459, tel. 043/630398, US$13) is probably the next best of the cheapest.
On the west side of the Plaza de Armas, Gran Hotel Müso (Valdivia 222, tel. 043/313183, US$61/82 s/d) is a business-oriented hotel that offers discounts up to 25 percent off these rack rates for payment in cash—a better bargain than the usual IVA discounts. Just off the plaza, Hotel Mariscal Alcázar (Lautaro 385, tel. 043/311725, www.hotelalcazar.cl [1], US$74/93 s/d) has 60 renovated rooms, the quieter of which face the interior garden, and a well-regarded restaurant.
Links:
[1] http://www.hotelalcazar.cl