If you need 5 year (20 pts), you need to report about 5k self employment income for 5 years. The price is 25k*16% paid to SS (since it is self employment), which is about 4k, and whatever fed and state tax due to that amount, say a total of 15k in worst case.
Now, if you have 5 years reporting 100k earned income, you will probably get $500 a month when you reach retirement age (better use the SS website to put in the real numbers to see how much you will get). If so then it is worth to buy the points since you are likely to live more than a few years collecting the 500/m.
