I've got an XD40SC and I have the same issues. There's no deals. Keep your eyes open and if you get to the same point I'm getting to, pay full-pop and be done with it.
It can sometimes be hard to find good prices on well made or factory pistol magazines.
Other than buying S-A XD pistol mags directly from the company, I'd check these online sources. Popular after market firms like Pro-Mag or MecGar may stock the XD compact/sub compact line.
NOTE: I heard the newer XDm pistols require different pistol mags so be careful before you buy. The XDm type do not fit with the XD models.