I have a lot of experience creating Augmented Reality applications, and have recently been working extensively with Unity. For your budget I would envision using Metaio or something similar for Image Tracking. Unfortunately, Metaio charges $3,500 for a license for their product, so for this budget we would have to use their sample version, which would place a watermark over the image, which is fine if this is to be used to demonstrate feasibility for a client, but obviously not acceptable for a finished product. Let me know if you would like to discuss further.
Thanks,
Chris Howell
etherVision, inc.