It's actually 3% of your basic military pay. If you are in a 6c position you military time does not reduce the amount of time you must serve. If you are in a non 6c position it all counts the same. As far as the interest rate, I have no idea, but one day I did figure out how much it would cost me for 10 years and it only came out to about $6000. If it were me I'd save me butt off for the first couple of years to try and hit that mark then pay for the time outright.
On a personal note I noticed the Reserve technicians receive the 6c coverage. I always thought if the reservists, who are doing the same job as the active military, receive 6c then why can't the military purchase their time as 6c. Even if they have to pay a higher rate than the normal retirement. Or be given credit for time so they could hire into a 6c position at an older age.
But when a long train of abuses and usurpations, pursuing invariably the same object, evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new guards for their future security.
Translation for the intellectually challenged: If the government screws the people too much, it is the right and duty of the people to revolt and form a new government.