I just found an interesting bit of information for you on income levels in St.Gallen. This information is published by the city tax office according to taxable income for individuals in St.Gallen city. Just to confirm though, when you look at these numbers, please don’t think that the majority of people are poor :-). Actual incomes are normally higher. Have a look at your annual tax summary to figure out how you are doing comparatively.
For reference, there are around 78,000 inhabitants in St.Gallen (29% of whom are foreigners).
At some point during your time in Switzerland you are going to have to fill out tax return forms. When you do it will depend on your permit status, salary level, nationality and length of your stay. Even if you speak good German, this can be confusing and it can be hard to know if you are really filling out the forms right and if you are taking advantage of all of the relevant rebates.
The best thing you can do is get a tax advisor. For filling out the tax returns forms for easy cases, this will cost only about CHF 200 each year. It will probably save you a lot more than that on your tax bill, and will save you a lot of stress.
I only know of one fluent English speaking service in St.Gallen but I am sure there must be others. However the following provide an excellent service: http://www.scriptoconsult.ch