"Hipotecarios" is an adjective, so that word is never used alone. It always goes with a noun. (eg: "Préstamos hipotecarios", "Créditos hipotecarios", "Seguro hipotecario").
This is the word you would use to translate "mortgage assurance", "mortgage loan" and such.
"Hipoteca" is the one and only proper word for "Mortgage" (alone) in spanish.
That said, the name is still brandable and, as you saw, it is used to develop mortgage related sites (like the one in the .com).
However, for spanish .mobi names I dont like brandable expressions (yet) and I would say only generics achieve decent prices at this stage.
So my appraisal is something in the low $XXX range.
Good luck.