I cut my teeth on an 80's version of Differential Equations and Their Applications: An Introduction to Applied Mathematics by Martin Braun . Amazon lists copies as going for under $50. Perhaps someone from this millenium can say if the new editions are still good. I liked the anecdote about the Tacoma Narrows Bridge, and found many of his examples well motivated.

Gerhard "Ask Me About System Design" Paseman, 2010.04.15