I'd largely agree with DQ. Certainly a teacher who's received extensive training in teaching methods is much more likely to be alert to her student's needs and to know how best to address those needs: a major reason why Profesora García has helped me so much, I believe, is her teacher's training (she holds a baccalaureate in education); though her competence in English might be good for some students, her thorough knowledge of Spanish and of how to convey that knowledge to a student was what benefited (and interested) me most.
I did study for some weeks in two language institutes, the CUI and the Laboratorio de Idiomas, and found those teachers very good; but the participation of the other students (three or four others in the CUI, a half-dozen others in the Laboratorio of the UBA) did not assist me. Perhaps having Argentine friends and cousins, all well-educated, has been more beneficial.
Bottom line: different people learn in different ways. A trained teacher tutoring a single student is, I think, the one person most likely to help a student at whatever level of competence. If one is serious about learning the language well, it's worth any reasonable price to study with the best available (I don't know what current rates might be, but I doubt that many teachers would charge much above the equivalent of ten or fifteen American dollars for an hour's tutorial).
