Alright, I'm going to college in the United States right now, and minoring in Spanish so hopefully I can help you out some with this. Alright, first as to whether a career as an interpretor is a good career salary-wise, this is what I have found by doing a little web-research:
Mean Salary Potential (projected): $70455/yr (Economic Research Institute)
Average Income (2008): $51,340 (The Career Project)
Based on that, I think it's a pretty good job. That's a higher salary than teaching, just to put it in relation to something. I also imagine that your salary would fluctuate significantly depending on your experience in the field, as well as location (there is probably a higher demand for Spanish translators in the Southwest US than in Canada).
I'm not exactly sure what you're describing with the "mother language" and such. If you are asking if it's best to be fluent in both Spanish and English, then I would have to say that yes, it is a must.
At this point, I would take as many high school Spanish classes as you can. Mastery of both the spoken and written language is important if you want to be a translator. Additionally, if you can find a way to work with native speakers, such as volunteer with a local English language learning center (if you have those or something like it...). Having a good Spanish background can only help you once you get to the college level.
As far as college classes go, I can tell you that I'm taking 21 credits of Spanish, which adds up to about one Spanish course per semester (keep in mind I am minoring in it, so you would have double that most likely). If you want a specific look at what exactly you're looking into here, I would pick out the universities you are considering and scan their website for a course prospectus or program information. That usually contains the type of information you seem to be looking for. This is an example of what you're looking for, taken from my university's website.
If you are having a hard time finding it on the web, I would recommend calling up the university and requesting the information. I know a lot of colleges will send you packets of information if you are interested, you just need to let them know.
Finally, as to the cost, I have to again recommend that you check out the college's website. It is almost always listed on there. If you can't find it anywhere, you could try using this.
Hope that helps some. If you have any other questions or need someone to talk to about any of this, feel free to contact me. I would be more than happy to help.
Take care.
-Isobel
