matrixshaman is correct, or should I say Google is correct! The bills that are replaced are given a new serial number with a star suffix, they will keep a talley of damaged notes then do a run of replacement or star notes starting with serial number 0000 0001 through how ever many need replaced from that mint. The main reason they don't use the original serial number is it would be too time consuming to have the presses reset for each number.
The value of a star note depends on a number of factors, first of course would be condition, a nice crispy bill with no folds or creases is most desirable, a limp bill that resembles tissue will most likely carry just face value. Other factors would be the age (series print) and how many were printed for that series, and the actual serial number, a low number is more desirable. A brand new, uncirculated five dollar bill of a new print may carry a value of up to 150% of the face value, ballpark estimate. And yes, bill collecting is another hobby of mine!