Do not place a bid and negotiate your own terms after. You will be reported to Freelancer immediately if you do. So place the bid that you think the job is worth and I'll award you the job if it's within reason.
I am willing to pay up to $35 commission per paying subscriber who joins the Senuke program through my affiliate link. For those of you who knows what SENuke is, it's probably the best IM tool in the industry and comes with a 14 day free trial. Normally 1 out of 10 customers who join the program on a trial basis go on to become paying customers. And this is where you come in. Simply bid the amount that you are willing to accept per subscriber or the amount to reflect the total number of subscribers you think you can acquire.
So if you think you can acquire 5 paying subscribers, bid $175 which is the most I am willing to pay for it. And for 10 paying subscribers, the amount will be $350. If you place a bid in the value range of 5 paying subscribers, I'll pay you as and when they become full members through "milestone payments". And with SENuke the affiliate sources are very easy to track. You just have to tell me the source url of where the leads or sales are generated and the affiliate sales report does all the rest.
Good luck with the bidding!