If you want only to test if the camera is centered, the cheapest way is to not buy any cable or any monitor. Mount the camera, fix it in a position you think is perfect then start to record. Then play that record on some PC. After some tests you will find the perfect position.
Another method is to look for some friend which have some device with AV In. If yes, you will need only the cable.
But if you still want a monitor first check if your camera have playback mode so you will use it not only for the live video provided by the camera. If a camera doesn't have a playback mode a monitor is not really needed. Think also where you will mount the monitor. But if you still want one you can searc on ebay for TFT Monitor and you will find with prices starting from $17. But you will need to wait more and your camera is still not centered.
enjoy,
Mtz