Inexpensive Motion detection/monitoring a doorway

I just moved my retail store into a new location. Many benefits to this,
most of them financial, but the cool thing is that I once again have an
office. With a door and everything!

Which causes a problem. I cant see the entryway when I'm in the office.
In the past, I've used a simple wireless door chime. But at the new
location I want to leave the door open.

I'm thinking of a webcam with motion detection. But I do not want to spend
much money. I'm thinking $50-$75 is my price range.

There are so many options I'm getting a headache. But I don't need fancy
(read: Dont want to spend a lot of money). A wireless Web cam in a
different room plugged into the wall for power. I've got extra old systems
that could act as a server. I want to be able to view from inside my
office and I'd like a sound to play when somebody is detected.

Even an old fashioned "electric eye" would work. But I've not been able to
find something like that. They have to still exist, but where?

Any ideas?

The thing I've learned over the last few years of owning a store is that
money is no longer an abstract concept. It is right out of my back pocket!

Bob Crothers
www.BJsBariatrics.Com
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"What part of 'Thou shalt not…' didn’t you understand?"
- Billboard in Las Vegas