cracked laptop casing

If you wish to do it yourself, you will need to purchase an exact same Casing and completely disassemble the laptop and reassemble it in the new casing.

This will be a lot of work, and may prove to be quite challenging if your technical expertise is not up to par. It is easy to make a mistake, forget to hook up a connection or simply break one of the fragile ribbon cables.

Is it out of warranty period, will the warranty even cover the damage if it is caused by user mishandling? Service will likely be expensive too, costing 1-2 hundred dollars or so at least.

I would suggest -
1. If your laptop is fully functional and you can live with it then leave it be.
2. If you feel your technical skills are strong enough, you should purchase a casing off ebay (or a broken/damaged unit for spares/repairs) and attempt the repair yourself while carefully following a disassembly/assembly guide.
 
You can probably buy a new casing, or possibly even just the specific part of it that you need on Ebay. Of course that would require disassembly of at least part of the laptop, which should only be done by someone who knows what they're doing. Alternatively some epoxy or something similar could be used to reinforce the crack, it all depends on how bad it is and how good you want it to look.

http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xdell+inspiron+15+5000+casing.TRS0&_nkw=dell+inspiron+15+5000+casing&_sacat=0