I would say $6 is a bare minimum. With postage running $5.25 for the small flat rate boxes and $5.75 for the padded envelopes, my shipping costs were covered. I used my laser printer with the peel/sticker paper for the labels and those have a cost, so I'd say I got pretty close to breaking even. I might have made 3 or 4 dollars. I certainly didn't lose money. I wouldn't mind paying $7 to participate but $8 per person feels a little steep.
I can envision some people charging that much, or even more, if they are offering something well above and beyond being a host for a regular swap. If it was something highly specialized or the host was especially sought after and well known as an excellent host, they might be able to command a higher entrance fee. But those circumstances would be unusual, I think.
