I agree. You should nearly double the cost. Towers could eat up over 100k easily. Ground system will be another huge expense.
Then there's the cost of the antenna phasing system, transmission lines, sampling lines, phase monitor, STL, remote control, audio processing and ATU's.
I won't even go into the engineering costs for system design and proofing.
You'll be lucky to build the place for 600k.
AM ain't cheap.