Rodger,
There are numerous fees that will need to be paid either to set up the deputyship or for the ongoing supervision/audit of accounts etc. Any fees do not fall to you to be paid but to your mother in law, as does the cost of applying for deputyship in the first instance. This is something which should be included in the wording of the deputyship order that you ask the court of protection to approve. You say that neither you nor your mother in law have much money, so my first question to you is to consider why you feel deputyship is necessary. I can understand it being necessary, for example, if there is real estate involved or if the person has a large amount of savings that would be required to fund care etc. If your mother in law has less than £14,500 in the bank and no property then any care fees, for example, would be met by the local authority. In this type of scenario, say, where someone's only income is state pension then you can manage their affairs by becoming an appointee for the receipt and management of pension payments from the DWP.
As we don't have more specific details about your mother in law's position it is hard to discuss deputyship in any great detail other than to give you an indication of the cost of obtaining and maintaining it which saffie has done already for you.
If you want to come back and discuss this some more before sending the forms off then there are many on here who will share their experiences with you I am sure.
Fiona