Unfortunatelly Aspose.Tasks does not read resource assignments CostRateTable values from MPP files now.
I have created a new issue “Read resource assignment’s CostRateTable values from MS Project 2003/2007/2010 MPP files.” with ID = 19912 and linked it to this forum thread. We are planning to resolve the issue in our October release (end of the October).
You can find the information that the values reading is supported for XML format only now in our documentation here. Sorry for inconvenience.