I approach mailers like solo ads. If an upgrade cost $10 I expect to get 7-8 sign ups. 10 sign ups or more for $10 is a steal. 2 signups for $10 or $5 per lead is a no-no. Results vary a lot so sometimes I use an average like X amount of signups for x mailer upgrade budget and re-balance by downgrading the ones that are not performing as expected and upgrading the ones that are giving me good results as a free member. OTOs also affect results. At regular price you may find yourself paying $3 per lead when you could have been paying $1 per lead if you had taken the OTO.
Just my 2 cents..
-Luis