tear down vs rehab

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are there any quick metrics to determine the best strategy?

example: where i live, its a neighborhood 2 blocks from the campus of a big 12 university. the neighborhood is mixed, rentals, single family, big old houses chopped up into multi-unit rentals.

recently, in the last 3-4 years, smallish rental houses on bigger lots (~1000 sq ft houses on 8000 sq ft lots) have been bought, torn down and rebuilt as 2500+ sq single family residences. or rehabbed, with additions, turned into single fam. the lot five down from me was turned 2 years ago from a 1800 sq/ft duplex to a 3300 sq/ft single family. sold for 400K. i have the same lot size as this one.

one example half a block from me: 800 sq ft house bought for 70K in january, torn down, now a 2600 sq house is built and not quite finished but should be on the market next month or so. this builder offered to buy my place 2 years ago, but i told him i'm not interested. i was still in college and didnt want to sell.

i am guessing construction costs are about 80-100/sq ft (what new construction is out on the outskirts of town) that would put this guy in at 70K + appox ~10K tear down cost + 4500 permit fee =84.5K plus (208000-260000)

so total cost is 292500-344500.

older houses that are in good shape go for 120-135 sq. foot. my place is 1400 sq ft but i'm second owner (80 yrs old) and have made modest improvements like central a/c,heat and new appliances, etc. i've had at least 6-8 people tell me they would buy my house while i was still in school, but never wanted to sell. i had a re invest type guy say my corner lot was worth 110K without the house.

any ideas what an as-is value for my place would be?

thx,
blangtang