How to use SAS Expander with RAID card?

I've doing some researches since few days and I haven't really got an answer to my question.

I am in the process of building a huge file server for the company I work for. We bought a server case NORCO RPC-4224 4U rackmount server case with 24 bays and drives. Now I am stuck at the point where I have to choose the RAID card. I saw a RAID card from LSI that offer 6 internals mini-SAS 8087 ports (6x4) and would work perfectly with my build. However, it's not cheap.

I have been reading about the subject and I found something called SAS Expander. They are cheap. HP is selling an expander card with 6 mini SAS 8087 for 225$ (instead of 1150$ of the other one).

Now my question, how would I link an SAS Expander with a RAID card ? Do I need to a buy a RAID card with only 1 external 8088 port and link it with the other card ? Could someone clarify this for me because I haven't found any good documentation on it.

Thanks you.