As somebody who knows many current and former astronauts (lived a stone's throw from JSC for 20+ years)... I can assure you that astronauts picking up money for appearances is fairly uncommon. A majority are incredibly low-profile, publicity-wise. If they show up to an event to speak, it's almost always because they personally feel compelled to advocate for research, education, the space program in general, etc.
In addition, the demand for astronauts is astronomically low :), so the supply of people wanting to do it is very high compared to demand.
Think about scuba diving instructors. They don't make that much money because it's such an awesome job. If I could make 150k a year doing that, I would quit my job in a heartbeat, move to Hawaii and never look back. However, they don't because if they started paying that much, a million other people would do the same, flooding the market with scuba instructors which would shift the salary back to what it is now.
Nobody applies to be an astronaut to get rich, but for a well-educated person $150k in pay and benefits is absolutely possible, which is a great paycheck, especially in Houston.
The GS grades are not about education, but job responsibility. The grade for a given position also depends on the agency you're working for and where. An engineer with the AF working for ACC might be a GS-11, but a GS-12 if they switched to AFMC, or GS-13 if they did the same job with AFMC at another base.
I misread the GS site, you are correct.
Still quite a bit above the median in terms of salary, which was my only real point (and that if you are primarily motivated by money, advanced degrees and Federal government employment is probably not in your future, anyway).
As for post-astro careers: my understanding is that a number of astronauts have gone on to academia.
From http://www.federaljobs.net/salarybase.htm#SALARY_TABLE_2015-... it sounds like civilian astronaut salaries then range from $51k to $112k per year. That feels pretty low, but presumably astonauts assigned to a mission can make money through public speaking, etc.