Dick, I'm with others - esp. Leadfingers and Gervase 24 Jan 09 - 06:00 PM .
You bleed for you art and, thereafter make a difficult living or become famous and make a load of bobs.
If a member of the audience wants to buy you a drink, or stick a fiver in your pocket - great ! - but - it won't become an habit in English Folk Clubs.
Why should it ? you have pitched your Fee at a level that you and the organiser have felt is reasonable.
If circumstances demand that you require a "top-up" to the Fee then my previous sentence is even more succinct.
If you can't make a reasonable living , then you may need a re-think the whole thing, or in place of the agreed Fee, tell the organiser you will accept - the contents of a "bottle" (glass) collection [Dangerous]
You have chosen a difficult Road ,and good luck to ya',but in-house busking , and Folk Club performance are two entirely different matters.
I don't like your proposition and hope it never occurs in English (or British ) Folk Clubs - people as individuals are entitled to pop some money in you top pocket if they so wish to do so , but if I find that after paying an entrance Fee, I am asked to make further contribution to the performer's coffers - it will be greated by me with a firm Feck off.