You should never choose accountants based on how much they charge. Choose them based on how much money and time you think they are likely to save you but you will obviously need to know what their charges are from the onset.
Some of their fees are based on hourly rates, which vary from one accountant to another depending on on who actually does the work for you. Some tasks like audits, are usually charges at a set amount. Make sure you find out beforehand what the annual cost of the service is and if you have to pay all in one go or if you can pay in installments.
The fees you pay can be significantly reduced if you plan yourself properly and spend less time with the accountant. Making sure your financial records are all up to date and and properly organised can cut down on time spent. Ask your accountant to give you a checklist to help you collate all the required information which will save you both time and money.