I currently run a website that generates income based upon purchasing
a yearly membership.  My site generates about 100,000 visits a day. 
These visits are from my members - so they are not different visitors
on a day by day basis. but are 100,000 unique visitors in a given day.
 I currently do not accept advertising on my site.  So my question is,
how much do you think I can generate via ad space per month?  For
example, having a bunch of advertisers' banners rotate on various
pages.  The reason why this question is important is that my visitors
appreciate that my website is ad-free.  However, it wouldn't be an
issue if the website members area was free. So I would actually be
abandoning my sales model of charging my users a membership fee in
exchange for generating money through advertising instead (only if the
amount I generated via selling ad space is higher than what I make by
charging a membership fee).  Also, how does one go about finding high
profile advertisers?  I know there are websites like adbrite for
selling ad space... any other suggestion?  My site is in the sports
industry.  Thank you in advance.

It's depend on which types of ads you want to put. If you want to put
adsense then you may be earn 200-500$ per day.

If you want to put CJ then you can earn almost 300-1000$ per Day.