I'm doing a Charity "Paint and Take" session for a fundraising drive at work; how much do you reckon I should charge?

The place I work at is doing a fundraising drive for a local hospice, where we a bunch of us do mini-fundraisers throughout a month. Last year I did a sponsored painting marathon for those of us on staff who paint minis, but this year I'm doing a "paint and take", like you see at conventions or Warhammer shops. Staff who wanna do it donate to book a time slot, I teach them how to paint, and they get to take the mini they've painted home with them afterwards.

The thing is, I'm not sure what's a reasonable amount I should be asking for as a donation? They're leftover models from my sprue pile & I'd be shocked if they use up an entire pot's worth of paint between them, so it's costing me peanuts to run, and I'm not expecting a lot of people to sign up, so I'm thinking £6 or £7 a go. Does that sound like a sensible amount to you, as Wargamers yourselves, or should I go higher?