bhāvanātvaṃ nāma bhavituḥ prayojakavyāpāratvam. tatrārthabhāvanāyāṃ bhavitur jāyamānasya svargādeḥ prayojakavyāpāratvāl lakṣaṇasaṅgatiḥ. śabdabhāvanāyām api puruṣapravṛttirūpasya bhavituḥ prayojakavyāpāratvāl lakṣaṇasaṅgatiḥ.Which, I believe, can be translated as follows:
The fact of being a bhāvanā consists in being the activity inducing (prayojaka) something which is [through that] about to be brought into existence. Among those [activities], the definition suites the “arthabhāvanā” insofar as this is an activity inducing heaven and so on, which is brought into existence, that is, born. The definition also suites the “śabdabhāvanā” insofar as this is an activity inducing something which is about to be brought into existence [and] which has the nature of a human activity (vyāpāra).
1) prayojakavyāpāra is here equated to bhāvanā (and not to X-bhāvanā), hence, one does not need to presuppose something like śabda-bhāvanā=prayojakavyāpāra.
2) the symmetry between the two holds on only insofar as the bhāvanā part is concerned. The first part of the compounds is not compared.
3) artha is possibly thought of as hinting at svarga.
Obviously enough, this regards only Kṛṇa Yajvan and not necessarily Kumārila. I have already collected different definitions by other Bhāṭṭas.