Now that might be doable. If the software supports it.
Sig Sizes - Morrus it is doable with Vbull, I know several sites set up that way.
Yes the comparrisson to extortion will be there. After all you deal with the wave of 'NerdRage' with us.

You won't please everyone, don't try, you'll fail miserably if you do.
Set our plan and stick with it.
While I might complain / *itch about it, I would rather see the site succeed than fail. The site itself has value to me even though its not a value that I consider paying for.
One thing I take from this in suggestions:
Restricted Sigs - Agreeable
No Sigs - Not agreeable
Another idea to use:
Control Panel functions.
Think about it, everyone here loves being able to increase posts per page, etc.
This makes for a nice perk to offer to subscribers. Think about it, it takes nothing essentially away from the functionality of the site, but increases usability for those of us with higher speed internet service.
There are lots of little things you can do such as that before you effect serviciblity of the site to 'freeloaders' like myself.