Hotel in SD
Ok, here's a little place in the colonial zone that's supposed to be good, though I have never used it myself. ( I haven't been back to SD in over a year ).
At $250 pesos/night ( $15 ), it might be a little rustic, but the source is trustworthy.
Contact Maria Perez at
Ciudad Colonial
Calle Hostos 299
809-688-9192
I think she's French and the management is German.
If not that then I've heard the Palacio also in the colonial zone is good at more like $50 / night. I'm sure that should suit your needs. There's no reason to pay more than that.
I'd stayed at the Napolitano on the Malecon for $75 and was told I overpaid. Probably true.