Any gap between a crown /cap and adjacent natural tooth dictates examination of the area of concern checking for dental caries in the adjacent tooth or crown breakage or in some cases over zealous flossing can create an unnatural gap. A filling or a new crown shud solve the issue in most cases

There is nothing such as permanent denture just to clear your doubt. On the other hand if u want to replace a single tooth especially in your case as it is the lower right no 7 tooth, there is only 1 permanent option ie. Implant. But as your complaint suggests if there is only an issue of gap between your cap & the tooth, then with proper evaluation if your root canal treatment is not re-infected we can just change the cap and resolve the issue rather than replacing the whole tooth. The replacement of a cap can cost you between 5000/- to 10000/- but the replacement of the tooth with the help of an implant will cost you between 30000/- to 50000/-