How to calculate walls-in (H06) insurance coverage

Can anyone advise me how to work out how much ‘walls-in’ insurance coverage I should have for my 1,000 sq ft condo apartment? Is there a standard ‘rebuilding cost per sq ft’ number that I could use? My current coverage is $150K which works out at $150/sq ft. That seems low to me. I know I could ask my insurer but I would like to get some sense of typical costs from disinterested third parties (i.e. Forum readers) before I talk to them so I have something against which to compare their answer. Thanks.