How much does a passenger-to-freighter conversion cost?
Companies converted 87 passenger jets in 2018 ... Direct conversion costs for the 87 aircraft totaled \$641.5 million ... The direct cost of converting a Boeing 767-300ER amounts to some \$14 million, compared with \$15 million for an A330-200 and \$16 million for an A330-300 ...


Source: Freighter Conversion Market Totaled Nearly \$1B in 2018: IBA, ainonline.com

2018's average is 7–8 million USD per conversion.

Based on this source, an A330-300's heavy (D) check costs 3.8 million USD (adjusted for inflation from 2010 to 2018), and given the quoted conversion for the A333, it's ~4 times as much to do the conversion.


  
  Source: Boeing 747-400 Passengers to Cargo Conversion, Israel Aerospace Industries, YouTube


Of the changes:


The fuselage is cut and reinforced to allow for the bigger door
The floor may require reinforcement
Passenger environmental control systems are removed
Cargo fire suppression is installed
A 9G crash net is installed.


More in this A330-300/-200 P2F brochure (PDF). And then there is certification.