Even though I might not agree with it... I could see the Steelers signing Foster for relatively cheap compared to Starks. The primary reason is the position... Foster has been backup that has played decent because of injuries. Starks on the other hand is a starter in this league and he plays the most expensive position on the o-line: LT
For financial reasons, they might sign Foster to a cap-friendly deal and let Starks walk. They invested high picks in both Gilbert and Adams. The Steelers obviously planned them to be starters and that could spell the end for Starks in Pittsburgh. Depth is a whole seperate issue. You don't draft a 2nd rounder (i.e. Gilberts/Adams) for depth. You also don't keep Starks for depth (to expensive and he's starter material).