I think it's OK to have visits paid by schools that you have little chance to attend. You paid for the application fee, they want you to have a visit, it's fair. No worry about the cost, I think they probably use others' application fee to cover all the expenses during your visit.
Personally speaking, an onsite interview will let you have a better chance of getting offers... And it's also a better way to know the school, program and PIs in person.