This app was mentioned in 3 comments, with an average of 2.67 upvotes
The motion detection app I use is Red Alert movemebt detector. I'm on my Chromebook, so that link may not be right for your phone. I have an older Android phone and had to download the apk for this app to work. By older, I mean it's like Android 2.3.4 or something.
For tracking, I use Where's my Droid. It's proabably no longer the best, but when I set the phone up awhile back, it was one of the only apps that was free and reliable. And, then there are tasker profiles to lock phone, send location, take picture, etc. in case I need them.
Most of the motion detection apps are camera based, so if Red Alert doesn't work for you (I'm not sure if it's still supported), find one that works with the accelerometer as the camera-based apps won't do much if the phone is tucked away. I've wired in a switch to the phone charger so I can turn it on and off if I'm worried about the charger draining the battery. If I'm gone for any length of time, I leave the battery tender connected and the power on. But, a phone charger will kill a battery in a couple of days if you leave it always-on with the bike not running. There are people that hack in a solar panel/charger in their pillion seat and this could feasibly take care of that problem. My bikes are garaged or in secured parking 99% of the time, so the solar option doesn't work for me.
These apps work for my needs and I haven't looked for all-in-one apps in awhile, but there are probably some out there with all of these options.
As for prepaid plans, I live in Asia, so I can put a couple of bucks on the SIM card and it'll last a few months. In the States, maybe you can find something similar, but the easiest might be adding it to your family plan for no extra cost (if that's still a possibility). Or, maybe Wal-mart plans? I don't know.
In addition to all else said here, get a cheap Android phone and a cheap pre-paid plan, preferably something you pay for X amount of minutes and X amount of data.
I use an app called Red Alert Movement Detector. It's no-frills, but gets the job done. I keep it plugged in and under my second seat. If my bike is moved (you can set the sensitivity), I get an SMS and a phone call alerting me. You can also set it to sound an alarm (as a deterrent more than anything because it'd only be as loud as your phone).
I use this on top of a chain and disc lock when my bike is parked. I also use a tracking app which will text bike's location.
Ideally, there would be an app that does everything: movement alarm, GPS tracking, take pictures, etc. But, I haven't found one yet. It could be done with Tasker profiles, but I'm too lazy to slog through setting it up.
This is exactly my setup.
I have an old HTC android phone with an AC adapter wired into my under seat storage. I use Red Alert as motion detection. I set it, and if the bike is moved I get a text and a call. You can adjust the sensitivity and you can also have it sound an alarm. The hardest thing about it is setting it and getting the seat back on before I trigger the alarm.
On top of that, I have Prey and another tracking app for the chances that the bike is moved/stashed somewhere.
I use a prepaid sim and throw a couple of bucks on it every once in awhile.
Ideally, there'd be an all-in-one app that could handle all of these functions. It'd be nice to be able to remote activate the alarm(s) with texts. (This could be done with Tasker, but I'm too lazy to work out all the profiles and troubleshoot. And, there was an app that did text activation that is no longer supported on my ancient Android.)