There are members from all over the world on this board. If you make allowances for one country, you really ought to make allowances for them all.
Since I'm pretty sure that's a logistical nightmare, it makes sense to me to simply pick one currency (presumably US$) and leave it up to the end-user whether they want to pony up the membership costs.
You want all the perks that comes with a membership? Pay for it. If you're content to come here and enjoy the free bits of the site, do that.
I'm also thinking one membership fee is far better than a scale, but that's just me and my desire for simplicity. The difference between $10 a year and $20 is pretty negligible, esp. when you stretch it out over the year.
That's about .03 cents to .06 cents a day. This site is, IMO, worth way more at it stands now. Add the various features we're talking about here, plus the possibility of some good fanmade product available for download regularly, and the value shoots through the roof.