In a previous life I used to look after an An124 ops and be in the commercial dept.
The Trent (in its traveling frame) or the equivilent GE and P&W engines will only fit the 124 or a front loading 747. We used to move GE engines from the east coast USA to Boeing field and we could only get 4 on at a time. Volume rather than weight being the issue.
A USA Eur round trip charter would normaly be around USD 250k per charter but this could rise if its a rush job and the acft had to be pullled off an already booked charter.... so the 500k quoted is not unusual.
cheers