The Siemens HiPath 3500 or 3550 would fall in that range easily. That system prices out well in the 15-50 telephone range. The phones would be the Optipoint 500 for digital or the Optipoint 410/420 for IP. They call them "real time IP systems" but they are really hybrid PBX's, they are not key systems and do not work like one. They can work in a key system mode, but they are really suited to businesses that need PBX features.
The 3500/3550 comes equipped 0x8x4 and you can add cards from there. There are 5 universal slots in the cabinet. The 3550 is wall mountable and the 3500 is rack mounted. Technically the 3550 can be rack mounted too, but you need to buy special brackets to do that.
Those systems come with a built in 2 port, 20 mail box, 2hr storage time voice mail system but you can also get the in-skin Xpressions Compact voice mail that has 8 ports and 70hrs of storage time.
In Europe they have an even smaller system called the 3300/3350, but we do not get that in the USA..
Let me know if you need more info!
Steve