Karen Margrave, i do not consider myself a developer even though i have bought houses, did the tear down and rebuilt, (keeping 1 wall) from the ground up.
That said, i can build for $100 per sqft. The big builders in my area can build around $67 to $80 depending on finishes. (permits, fees etc not included) I am guessing this is not a big builder, they would not mess with a small infill project like this.
Based on my quick calculations, it looks like a 12% net profit with an exit price of $650k, and 30% on a $750K exit. You will know better than i on what you think the property can do. If they can do this in 6 months, which if they have the experience, is a possibility,might be a deal.
this is all based on Cash on Cash. If there is borrowed money/ a pref. involved the numbers are less.
Investors are crazy right now. There is a lot of foreign money coming in and they are turning over every stone for returns. (land/builds included)
Hope this information helps.