How much money percentage should I ask for a project maintenance?

Question

Speaking about a project with a self constructed CMS and Web Site. Maintainable should be yearly and should include two fields.
Technical Maintenance
Tool update, Bug fixes, new modules implementation, design improvements, emergency plans against software and hardware failure.

Content Maintenance (updating videos, text, banners, communicating with client).

I was thinking about 30% from the original project price (15% Content Maintenance / 15% Technical Maintenance). What do you think?

Note. I do not know in advanced, how many hours i will spend. If they bomb me with new content and small features, I will have to do them. That's why i was thinking of a big percentage. Moreover, client is public administration so they pay well.

Sorry this is not really a programming question, but stack overflow is by far the quickest way to get a good answer. At least it is somehow technical related.

Answer 1

How many hours do you think you'll spend on it in a year? What do you consider a decent hourly wage for this type of work? Multiply the two numbers together, add a profit margin, and voila, you have your number.

Answer 2

Unfortunately, I agree with Paul. I do not think there is any cut and dry formula that you can follow for this question.

I would highly suggest sitting down and really thinking of the maximum amount of time that you think you might work on this customer and figuring out the cost that way. However, when it comes to talking to the customer about it, I would recommend rounding to the nearest 5%, that way your customer won't think you are just coming up with a random number. Seeing a maintanance budget of 25% for the year is easier to comprehend than 23.58201%

Good luck!