The actual filing fee is the least of your expenses. Please read on.
Joint owners of real property can file a 'petition to partition' in which the court adjudicates the respective rights of the parties. This is the equivalent of a property division in a divorce where the co owners are not married. The court looks at factors including contributions to the property, payments, sweat equity, and others. In a case like this involving a single family home the final order could award the property to one, with the debt or equity being split in accordance with a proportion the court finds "equitable", or the court could order a sale, with any sale proceeds or debt again split equitably.
I have done these. They take time and money. They can get somewhat complex and bogged down in procedural issues.
Therefore, it is recommended that you attempt a mediation or arbitration first.
Good luck with it.
This answer is provided for informational purposes only. Actual legal advice can only be provided in an office consultation by an attorney licensed in your jurisdiction, with experience in the area of law in which your concern lies.
As Andrew noted, this can get more complicated. To just file the case, for service and filing fees, it will be about $200. The court will actually require you to go to mediation within a few months after filing the case, and that would likely be the best time to resolve it. But because there are several factors that will go into deciding what is the fairest solution to the problem, it would be best to have an attorney.
I'd be happy to discuss this matter with you further.
The filing fee is $150 to file the action in Superior Court. http://www.courts.state.me.us/court_info/rules/fees.shtml. Most lawyers will charge hourly, although you might find someone willing to take the case on a contingency fee (percentage of the recovery) or on some mixed or flat fee basis depending on the amount in dispute (you say that the mortgage is paid in full and since the condo is in Cape you probably have a fair amount of equity in the property). You may also need to hire an accountant. You should get a free consultation and an estimate after your lawyer has had a chance to more fully understand the situation.