Thanks guys, had this same question and this is very helpful. However, I would like to put these "rules of thumbs" to test on a sample property and get your feedback. I 100% understand that state, county within state, building type, age, depth of coverage, etc., etc., etc. is going to make every property unique, but it would be great to come up with a rough rule of thumb that will allow a conservative estimate. Consider the following:
12 unit in somewhat rural Wisconsin, 6 separate duplexes on one piece of land, priced at $849K.
OM Insurance cost: $6200 (seems high)
Broker proforma: $3000
1.25% - 1.5% rule from @Juan Vargas: $10,612 to $12,735
$250/unit rule from @Gino Barbaro: $3000
The WI apartment deal isn't a traditional apartment building so I tried it on one more building. I analyzed, a 52 unit in Indiana going for $2.65MM:
OM actual insurance cost: $13,619
1.25% - 1.5% rule: $33,125 to $39,750
$250/unit rule: $13,000
Thanks for the feedback!