DITA is an XML format, so any editor or IDE that supports XML will work for you. Options with good XML support range from Eclipse (free) to Oxygen and Epic (several hundred dollars per seat). Of course, anybody who's comfortable getting up close and personal with the XML can use Emacs, vim, or Notepad++, too. (Don't laugh; I write all my XML and HTML in Emacs.)
Aside: MadCap Flare has some support for DITA, but it's not a complete solution. I use Flare for HTML projects and find it a good tool, but for DITA I would look elsewhere.
If you have several writers working on the same document suite, I hope you are already using source control. If you aren't, you should factor in those costs. Source control can be free (git) or not (svn, Perforce). If source control is already being used in-house, try to leverage that.
Doc tools and source-control products are frequently offered with both per-seat and enterprise licenses. The pricing break-even points vary, so you'll need to evaluate those options individually once you've decided on a tool set.
Finally, if you are setting up source control or a build system (to produce published docs from the DITA source) for the first time, you'll need to factor in the cost in someone's time to set that up and administer it. On the other hand, you'll no longer lose so much time to the costs of "informal" management -- someone has the document on the shared drive open so you can't edit it now, or two different people made conflicting edits to their own copies of a document, etc.