This is always a difficult question to answer since it depends on so many factors, including...
- what is it you want?
- how long does it take to provide it?
- how many cups of tea do you provide?
All that can be said is that you'll be able to afford it and it'll be a lot less than a new computer! And if you're not happy with the work done for you, then you don't pay anything!
If need be, ask Ken for some idea of likely cost before he comes; if you can describe the problem, he should be able to give you an idea of how much work is involved.
Ken's policy is that you should get the service you want at a price you think is fair and if either of these criteria is not met then he'd rather not charge anything!