1. A tangible computer readable medium including program code for providing message-based interfaces for creating a cost simulation, the medium comprising: a first set of program code for receiving via a first message-based interface derived from a common business object model, where the common business object model includes business objects having relationships that enable derivation of message-based interfaces and message packages, the first message-based interface exposing at least one service as defined in a service registry and from a heterogeneous application executing in an environment of computer systems providing message-based services, a first message for requesting creation of the cost simulation consisting of cost estimates with various cost sources that includes a first message package derived from the common business object model and hierarchically organized in memory as: a cost simulation creation request message entity; and a cost simulation package comprising a cost simulation entity and a property package, where the cost simulation entity includes a property definition class ID, and where the property package includes at least one property entity, where each of the at least one property entities includes a property entity property ID; a second set of program code for processing the first message according to the hierarchical organization of the first message package, where processing the first message includes unpacking the first message package based on the common business object model; and a third set of program code for sending a second message to the heterogeneous application responsive to the first message, where the second message includes a second message package derived from the common business object model to provide consistent semantics with the first message package.