what are you trying to accomplish with the extra sand/mud?
If your adding mud its to simulate a DSB, which apparently now isn't as beneficial as people thought it to be long run, and if not maintained can be bad for your tank. But mud needs to be changed out every once in a while. And it has to be set up correctly to work the right way, usually using a plenum system. But the mud will give the area the best surface area for beneficial bacteria to flourish. Ruling it out by saying "i don't use it because i heard its expensive" = the wrong way to look at things or tell people what they should do.
Sand your just basically adding more room for bacteria to populate and add some extra filtration. But with the very low sump flow rates dirt and crap tend to build up inside the sand usually giving you more trates and P04 than they remove in the long run without you stirring it every once in a while.
Or you can leave it empty. and just have that 5" of extra water which is cleaner for the system in the long run. You should have more than enough bio filtration in your system with the rocks and sand in the display and the basic maintenance, like water changes, running carbon/phosban, proper skimmer, ect.