What is reasonable will depend on the circumstances. The important thing is that whatever the decision is, the interests of the charity come first and you can demonstrate that.
The process for deciding the level of payment must be open and transparent and must not involve the charity trustee who is to receive payment or is connected to the person who will.
You could compare payment amounts with similar roles at other charities or the wider market place.
If the payment appears to be excessive, there may have been a breach of charity trustee duties, which would be misconduct.
Good practice 
- have a payment policy that makes sure any payments to charity trustees and/or connected people complies with the conditions set out in the 2005 Act
- establish a register of charity trustees’ interests
- obtain at least two separate quotes for services
- clearly minute the decision that paying a particular charity trustee or a person connected to them for services is in the charity’s interest.
If you are not sure if a charity trustee, or connected person, can be paid for services you should get professional advice.