basically 15 forts, considering you only can have 15 houses max. That yew large tower is worth more than a fort anyways. Grats, insane achievement
I would say start with a low number like 30 mill to get your foot in the door. Then start negotiating the price from there
With the recent housing crash and inrush of forts/castles on the market I'd start your offer at 15M for some of the low end locations. Get your foot in the door, you never know when the market will escalate again. 30m seems extremely high at the moment.