If you have any interest at all in military surplus or style firearms, consider some of the lesser-known options.
The semiauto PPSh41s and Suomis are climbing in price lately, but they're handy 7.62x25 or 9mm carbines, 30 rounds with the stick magazine, 72 with the drum. They're heavy by modern standards, which translates to low recoil, and their simple blowback design means there's little to go wrong.
The Uzi carbines are smaller, lighter, and can be had in the default 9x19, or in .45 ACP or .41 Action Express for more punch. Prices are surprisingly reasonable.
The layout of the PPS43 means that they're unwieldly long with a legal 16" barrel... but they're so cheap that even a brand new one plus a $200 NFA tax stamp would still be cheaper than some of the guns mentioned upthread, and with the short barrel and folding stock they're much more useful in close quarters. You can buy new semis in 7.62 or 9mm.