Mouldy Ceiling

The ceiling in our bathroom is mouldy because of poor ventalation. We have scrubbed it clean so many times it no longer will come completely clean. We have tried countless products, promising to eliminate mould, without success. We are worried about our children's health. They have bad allergies so it cannot stay like this any longer. We are now looking into what it would take to replace the ceiling and upgrade the fan/ventalation. Unfortunately our budget for this job is tiny (because of 6 months with no work). What can you suggest and how much would you anticipate a job like this costing?

Thank you.