No two opposition procedures are the same.
Some require a lot of work, some relatively little. Some require additional searches for prior art, some don’t. Some require interviews with experts and/or witnesses, some don’t. Some require witnesses to be present during oral proceedings, some don’t.
It will be clear that no fixed price for all opposition procedures can therefore be given. As a rough guidance, a low-to medium complexity opposition procedure should cost between 10000 € and 20000 €. There is not much difference in cost between seeking revocation and defending your patent.
Article 99 is open to discussing your requirements, both substantive as well as financial. If your requirements include a fixed price for a specific case, this can usually be agreed after a preliminary assessment by Article 99.