The Planet Offer was the online service of Cities XL. Players were able to sign up to the Planet Offer and join an online virtual planet. Each player could create up to 5 cities on any planet, and interact with other subscribers on the same planet. Resources were traded between players via a planet-wide virtual market, which was also accessible outside the game through a dedicated webpage. Subscribers could also visit other people's cities with their permission and walk through it with their 3D customizable avatar. Finally, a Blueprint lottery allowed players to win (or swap among each other) the opportunity to build Megastructures. The Blueprints for the Megastructures were available for a limited time after the acquisition, in which period the player had to start the project in one of his cities, or lose it.
There was also an event feature planned, in which players would be able to organize special limited time events in a city matching certain requirements (like concerts, or sports tournaments). This feature was never implemented, though.
Planet Offer subscribers had access to exclusive content, unavailable for offline players, like themed buildings, and the Bus network. Monte Cristo's original plan was to later make these additional features available for offline play for additional price (like a DLC), thus keeping offline play also developing, but always a step behind the Planet Offer.
The Planet Offer ended on March 8th 2010 with new subscriptions to the game ending Monday February 1st 2010. Read  for more information.
The pricing was originally as shown in the following table:
|1 month||2 months||3 months|
|€ 7,99||€ 12,99||€ 15,99|
|$ 9,25||$ 14,95||$ 18,95|
|6,95 £||8,95 £||11,95 £|
Later Monte Cristo reduced the prices, and then (just before they announced the end of Planet Offer) they eliminated all options except 1 month subscription.