Beater usually means used, old, ugly but still in good shape mechanically. I suggest Craigslist for $75-$200. Ultimately, the $ is a relative thing. I have friends whos' "beaters" are their $1,000 bikes compared to their $4,000 bikes.

You usually find that you need a beater when you face a situation where you are not willing to subject your nice bikes to (mine was the Arizona State University Campus).

I have two beaters.
1992ish Jamis Fully Rigid Mountain Bike. (ebay for $90 shipped)
1982ish Peugeot Road bike (thrift store for $40).

I have put additional money into both "beaters" but they are less likely to be stolen than my "nice" bikes and I cry less when they get left in the rain.

In my opinion, it is critical that your beaters meet two requirements. In my experience, these two requirements rule out most department store bikes:
1) they must be mechanically sound/efficient
2) you must enjoy riding them

If they do not meet these requirements, you will not use them and, ultimately, your nice bikes will be stolen or damaged instead of the beaters.