There are several ways to track costs in ReqPro, DOORs and other requirements management tools. All of the "big name" commercially available requirements management tools provide for user-defined requirements attributes (so you can implement your own cost tracking mechanism) and most include some form of template for cost management.
Caveat - I've seen more than one project death-spiral with huge overhead going into cost tracking and little into solution development because tracking the cost data became an end unto itself. In fact, there are often participants who have a vested interest in selling tools, selling support services, making cost tracking a full-time job, etc.....and all to the neglect of actually getting any productive work done. The best balance is usually found by figuring out the decisions that are/will actually be made, constructing cost attributes of requirements to support those decision, and implementing just this level of cost tracking...... and no more. This is a pretty good way to achieve cost management and not getting caught up in excessive cost tracking (no actual management involved) That caveat in mind, a general approach I suggest is to implement a few levels of cost management.