I am a marketing freelancer, so not exactly what you are looking for, but I do work with charities. I would say that with this sector maybe more than most what they are willing to pay is a big factor!
I have a contact who charges £400 per day (no VAT) for freelance fundraising work, but obviously that changes depending on who she is working for. Her background is more focused on major donors, she is SE too.
When I worked for a charity (£3 m t/o) we were used to paying between £350 - £1000 a day for consultant help depending on the need/expertise etc.
Could you work out what "you" would cost as an employee? I appreciate they probably couldn't employ a full time "you" but it might give you an idea of your 'value'.
I'd also have a poke around their latest accounts on the Charity Commission website. They might outline what sort of help they have purchased before etc.
I would imagine that they'd be looking for a fixed price job, so you'll need to estimate your time, which is always difficult even when you have done lots of freelance work. I find it always takes longer than you think! I think you just need to be honest, maybe estimate a little over as they'll probably try to negotiate down! Just don't get talked into payment by results!!
I use www.paymo.biz/ to track my time. It is free and has a desktop & app version. It helps you keep a realistic track of how long things take.
You aren't asking about this & may know about it...as with anyone thinking about going self-employed you also need to think about tax, NI, marketing costs if you foresee this becoming a big income stream for you. I work almost full time and I put 25% aside for all of that (earn under the higher tax bracket).
You can earn c£9K without paying tax, but you will still need to register as self-employed. www.hmrc.gov.uk/selfemployed/register-selfemp.htm
Not sure if that is any help! Hopefully a fundraiser will also be along to add their thoughts.
Good luck. Good fundraisers are always needed.