Thanks for the advice, how do you know how long jobs are going to take in a month and how many hours you are going to put into each service?
Sorry for the delayed response, been a Tad busy.
I would turn the argument around, by saying if you do hourly rates your clients will always question how you come up with the hours worked, you are also limiting yourself to hours you normally charge each week/month and any deviation will mean your client will question you.
It's not a question of how long a job takes, it's more what the Value to the client is. Example, when you service your car most dealers will quote a fixed price for a service, not the number of hours a technician will be working on your car. For you the value is 1) you know exactly how much it is going to cost and 2) The value you are going to get by knowing your car is maintained as recommended by the manufacturer. For the Car Dealer, the quicker they can do the service the more clients they can have and make more money - it's business.
In the case of a bookkeeper, yes you can keep the books manually say in a spreadsheet and the job may take you 10 hours a month to do, but if you get some bookkeeping software you can do the same Job in 2 hours. So the more efficient you are the benefit should be yours and not the clients. The client does not care whether you use a spreadsheet or some software, all they care is that the job is done, and they pay a fair fixed price each month.
So that in a nutshell is why you should always go for Fixed Monthly price. If you can do the job faster and more efficiently, then the benefit should be yours - it's business.
As to what you charge as a fixed price (as asked by OP), that really depends on type of client/ amount of work/ Value to client. I have clients that pay ranging from £250 a month to £1000+ a month. It all depends on size of company and what value you bring to the client. A tip is to price a client and have a clause in your agreement that fees will initially be reviewed 6 months from start and there after yearly. If you have priced too low, then after 6 months you can argue your case.
Hope this helps.