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.
Third party observations – Poor man’s opposition
There is a possibility for third parties to submit observations and arguments to the examiner relating to the (lack of) patentability of an invention. This is sometimes called a ‘poor man’s opposition’. The costs will be significantly lower.
A major difference to be kept in mind is that such third party observations are only possible before grant, whereas opposition is only possible after grant.
Further, there is a difference in arguments that may be used in both procedures. It is also quite possible that third party observations can damage the chances of succes in a real opposition. There is also a serious risk that examination is slowed down due to third party observations.
Third party observations are definitely not an instrument to be used lightly. They may do more harm than good.
But then again, if budget is too tight for real opposition proceedings, in many cases third party observations may achieve the desired goal that a patent is not granted.