can someone please explain this 50% rule to me?

[Jason Schmidt]

I have heard from people on these boards that I can expect the operating expenses of the house I purchase to rent out would be about 50% of what I can get for it monthly. I could be misunderstanding it, but how is this so?

For example: because of the tax rates here, we can expect to generally pay in rents about 1% of the home value. this includes insurances, taxes, interest, etc. SO - a working example:

If I buy a $100k house for $70k (assuming i can get in at 70% value), and I can rent at $1k a month, how would $500 of that each month go to operating expenses?