Roll up your sleeves (Score 1)

I just built such a thing over the last year. I bought PIR sensors and low voltage (i.e. telephony) wire. Hired an electrician to do all the runs through walls. It all ran back to a USB-1024LS which reads logic lines and plugs into USB. We already had a IP PBX running on a small 24/7 Linux box (which also handles DNS, DHCP, UPNP, etc.). Coded up a FSM to deal with all the alarm states, and wrote Asterisk AGI scripts to interface into the phone system (so dialing into Asterisk and having it talk to you with Flite is its UI). On alarm it can send e-mails and/or make phone calls. I beefed up the UPS so the alarm system and Internet feed can last a good long time even when the power drops.

So... roll your own is a very reasonable proposition if you have some basic scripting skills and some very basic low voltage wiring skills. I went with a wired system because I could see that all the Wifi-based ones could be easily disabled with a simple jammer. It also made it easy to supply UPS-backed 12V power to the alarm sensors, since I had that low voltage wire run anyway.