1. A computer implemented method of providing fee-based access to data at different levels of granularity of an abstract model of the data, the abstract model logically defining one or more abstract operations to access the data, the computer implemented method comprising: responsive to a request, performing, by a computer processor, an operation to access one or more physical fields containing at least a subset of the data, wherein the operation includes accessing the abstract model, the abstract model comprising metadata defining: (i) a plurality of logical fields in a form of a plurality of logical field definitions, wherein each logical field definition specifies a respective logical field name and a respective mapping rule comprising an access method that is executed to retrieve a corresponding physical field of physical entities of the data, each logical field definitions including a field-specific fee attribute; and (ii) a logical field category in a form of a logical field category definition specifying a category name and one or more of the plurality of logical fields to include in the logical field category, wherein the logical field category belongs to a parent category that in turn belongs to a grandparent category, of which at least one category has a category-specific fee attribute separate from any field-specific fee attribute of the one or more logical fields; wherein the operation further includes accessing the one or more physical fields via a first logical field of the plurality of logical fields and according to the mapping rules defined by the metadata, wherein the first logical field belongs to the logical field category; determining, by the computer processor, a first fee pertaining to the first logical field, by retrieving the field-specific fee attribute of each of the plurality of logical fields that are mapped to the one or more physical fields accessed by the operation, wherein a second fee pertaining to the logical field category is determined based on the category-specific fee attribute; and calculating, by the computer processor, a total fee to charge for accessing the data, wherein the total fee is a combination of the first fee and the second fee.