There have been a few threads about reasonable hotels in SD. Try searching and you should find about a dozen decent hotels mentioned under $60 a night or so...some half that. I have stayed at La Residencia on Calle Danae. Its in a really nice area called Gazcue. The owners are friendly and speak English pretty well. It's around $30 a night, small but clean rooms with a/c cable, fridge...walking distance to the conde, colonial zone, the malecon, the casinos, etc. For a few dollars more (I believe around $50 with breakfast), also in Gazcue, is Hostal (Hotel) Riazor, on Independencia, directly behind the Jaragua Hotel and Casino. A little more upscale and still pretty reasonable. The roof top pool may even be open...
Also, you may want to look into taking Metro Bus vs. Caribe tours, it's a few pesos more, but its a shorter trip (less stops), buses are a little nicer, and the stop in Sosua is closer to town - about a block and a half away from Rocky's