A friend of mine just got a job offer where compensation is base plus commission. Since she is in sales, this is pretty standard. Since she is the primary earner in her family, though, it can be a bit scary.
The company has given her two offers. The first with a low (especially for her) base, but a high commission percentage. The second offer has a much higher base (still not what she had been making) but a significantly lower commission percentage.
I ran the numbers to find the break even point, and I have to say, play the percentages.