The cost of Unusual Options Activity data varies depending on the quality and quantity of the data that you want.
If you want raw data from the exchanges like CBOE then you could easily be looking at a few thousand per month plus other costs. This is usually what institutions and data professionals use.
If you want basic filtered data then you can pay as much as $250/month. These are usually data resellers like Option Alerts. This level of data is used by a mix of retail and professionals.
The cheapest is $99 month or less from Unusual Options Activity services like mine and others like Cheddar Flow etc.