Listen to Charles. The man knows what he is talking about. How good are the demographics of this area if the lot rents are only $100? I wouldn't be surprised to find houses for sale int he $50k-$60k range, which makes this a bad bad bad area.
I'll also point out that your math seems off. If you assume a 30% expense ratio (which you should for most parks with direct billed city water/sewer), your valuation should be 61 x 100 x 12 x .7 x 10 = $512k. If you assume a 40% expense ratio (which you probably should since the lot rents are so low, as CHarles points out), your valuation is closer to your number: 61 x 100 x 12 x .6 x 10 = $439k.
Personally I would be scared to do this deal unless I got very comfortable with the demographics and actual market rents (do those other parks include water/sewer in the lot rent?) and comfortabel I could bump them $50/month (which is still awfully low for a place to live).