If it is a genuine issue, then company should bear the complete cost.
If it is malfunctioning because of negligence of employee like water spill or fallen, then you can ask employee to bear some cost.
You can even roll out a policy which roughly states as below:-
You may be given electronic gadgets like Computers, Laptop, USB Data Card, Telephone, Smart-phone, Sim Card or any other electronic device. It is your responsibility to use this equipment only for the official purpose and you are also expected to handle them with care. You should understand the importance of using the office equipment economically and ensure that you handle the machines smoothly & carefully as rough handling may cause damages that may end up spoiling the system. Failure to do so will invite the damages to be recovered from concerned employee.
In progressive companies, what employer does is an Asset is purchased with 50% - 50% contribution from both employee & employer and if employee completes particular tenure (say 3 years) with company, then that asset becomes Employees Asset and if he leave the organisation before the tenure (say 3 years), then employee has 2 option a) Pay the difference amount and take the asset or b) Employer pays the difference amount to employee and take the asset provided asset is in Good Condition else Employee will have to keep that asset.