Well the SA XD Sub 9 is already a pretty darn small gun so I guess you are wanting to venture into pocket gun territory. What kind of holster are you using on your XD? Those things aren't too tough to conceal with a good holster.
First thing I would do is check out www.impactguns.com . I think this is a great site for research. They have a huge list of guns. If it is not listed on the site then you are most likely going to have a hard time finding it anywhere else. Not only can you get a good idea of who makes what but what kind of prices different guns are going for. You may find prices elsewhere higher or lower but the prices listed should be in the ballpark. If technical specs for the gun are not listed just google the manufactures website and check out the info from there.
Guns off the top of my head that seem to fit in your category are Ruger LCP, Kahr PM9, and maybe a small Taurus or S&W revolver. There are other options but those are just a few.
Personally I would never even think to carry a .22. For one thing a lot of those short barreled small caliber(.22,.32,etc) pistols are not very accurate (not to say that bigger calibers are super accurate) and even if you did manage to hit the bad guy with every bullet in the darn thing he only might bleed to death after he killed you. Basically you will be under powered. I would not go with anything less than a .380 or a 9mm. Preferably a 9mm.
Some people will say "well carrying a .22 is better than not carrying at all" and some people will even say that a 9mm is underpowered. To each his own. If you want to carry a .22 then be my guest. I am not trying to be a smart ass it is just that caliber choice is a very personal issue and anytime it comes up someone is bound to get bent out of shape. I suggest doing more research. Check this forum for past threads on caliber selection,search the web, and check out some gun magazines at a store. Most gun mags are always talking about this caliber vs. that caliber.
If money is not an issue pop for the Kahr PM9. If it is then get a Ruger LCP if you can find one or a Keltec 9mm. Either way be sure to use some good defense ammo and practice with it to make sure it works well in your gun.