Ordinary or Partial differential equations? It may also help if you tell us what the outrageously expensive book is so we can better recommend a book of similar level. (You are not speaking of Birkhoff and Rota, are you? I know that one is about the price you listed.)

For ODEs, I'd recommend V.I. Arnold's book, it is a classic and under USD30 on Amazon for the paperback edition. Slightly more expensive is Philip Hartman's book, which also covers more material. I bought it many years ago, new, for under USD40, so I am not quite sure what's up with Amazon now that the Hardcover is listed for USD38, while the paperback for close to USD70.

For PDEs, McOwen's Partial Differential Equations: Methods and Applications is not bad at USD60, though its 1st order nonlinear system section is a little bit confusing.

(A bit of meta gripe: using multiple dollar signs in one paragraph seems to trigger math mode when it really oughtn't, which is why I wrote "USD" above for U.S. Dollars.)