What's an inexpensive substitute for Port wine?
Decent port is not necessarily expensive; you may find yourself spending more on the meat.  For example, Sandeman's Founders Reserve 750 ml runs about US$15 before taxes, Dow's Fine Tawny 750 ml is about US$13, and Graham's Six Grapes Ruby 750 ml is about US$8 (and this is just the start of the list).  These are, perhaps, not the connoisseur's choices, but they are hardly undrinkable.  Depending on the laws where you live, you (or your favorite wine shop) can order any of these online from a variety of sources.

Port has a unique character that is not easy to replicate with any other wine or fortified wine.  If you are trying to replicate a result, do not substitute.  If you really want something else that will serve the same role even if it will taste different, I would look to a Madeira, a cream sherry, or sweet Marsala (depending on your tastes).