We mostly only had enough volume for solo users, where we tested anywhere from $4-$20/month for an individual account. Found VERY little difference in conversion anywhere below $10, with a big dropoff thereafter. Pretty remarkable that conversions change almost not at all between 9.95 and 3.95/month.
Our volume for business signups wasn't super high at the time (that's since changed-- go RescueTime!), so we didn't fiddle much with that.