On Lincoln cents, all those minted in Philadelphia will not have a mint mark on the coin. These are sometimes called "Phillies".
On the 1998 Phillie Lincoln cent (no mint mark), the correct reverse design is where the A & M are close or touching. However, there were some 1998's, 1999's and 2000's that were minted with a proof design on the reverse. This design shows the A & M seperated or having a wide gap between the 2. Commonly referred to as a WAM (Wide A M). This design will also show the initials to the right of the memorial as being flared opposed to being straight or plain. A 1998 WAM (Wide A M) has sold on ebay for as little as $2-3.00 to as much as $25-35.00 that I know of, depending on grade. Hope this helps a little.