Compared to replacing a worn derailleur which is the case of the OP (see other thread). Just about any long cage Shimano RD will work on that bike, and function well with the triple up front.

Now if you are starting from scratch, the cost would be very competitive. But when you are converting an existing bike, that already has the rest of the parts in place, its a costly alternative. A new Shimano Deore RD can be had for about $25. Better models like a Deore LX can be found for about $35. A used RD would be less of course.

As far as gear capacity, go to the Sheldon Brown site, and run the gear calculator there. It is not uncommon for a triple touring bike like yours to have a 400% to 500% gearing range, or higher depending on chain ring sizes and rear freewheel.