The divorce process differs from the financial separation process in that you can claim for costs from the respondent if you divorce based on adultery or unreasonable behaviour.
In relation to financial proceedings, as a starting point, each party will bear there own costs. If you are applying for an order you will pay the court fee (£255) and nothing else unless you instruct a solicitor.
Your ex will incur no fees unless he instructs a solicitor. If he does instruct a solicitor he will be expected to pay for the costs himself.
You will not have to pay your ex''s legal costs unless you are unreasonable - the test for which is very high.
It would appear that the IVA did not make your ex exempt from being a buffoon.