100 signups of WHAT is the question
I'd much rather than 100 signups (per month) of $40-$60 accounts than having $100 signups (per week) of a $5.00 plan. It's not just price, it's the type of clientele. $5.00 accounts seem to come and go just as quick.
Of course, if you offer something FREE you instantly boost signups - and a FREE account can boost signups, however keep in mind the quality of the lead.
For forums - 100 signups quickly is attained by offering a giveaway - this is proven time and time again.
Advertising is what will bring in the customers - support and the perception of value for their dollar is what will keep them. I say "perception", as that's from the USERs end - you still have to actually offer value and follow through on their expectations!
Getting 100 purchases means you need a MINIMUM of 1000 visitors (and that's if 10% purchased). Your conversion rate is usually MUCH LOWER than that, but it depends on your target audience and the product you're targeting.