How much You can get by Selling your IIT JEE RANK ??

Shubham Tulsiani says

This is quite a common practice among institutes so I would like to address the trends too apart from the personal experiences and I'll also write on one aspect the question didn't explicitly ask  - offers before JEE

Pre-JEE (class 12th)
I had a couple of 'offers' -

1. Some people from Akash Institute came over to my place to 'gift' me books to help me prepare for JEE. All I had to do was - guess what - yes, sign a form ! I was amazed at how insistent they were and despite me saying I didnt need the books and wouldn't sign,  they made me keep the books. They then visited / called me about 5 more times to get my signature (until I finally returned the unused books on their last visit and told my hostel guard to not allow them back). I later learnt that they had been giving these 'gifts' to some of my (well performing) batchmates in Bansal Classes too and unfortunately a lot of them had signed up too. I was also once called up by some other coaching institute guy (maybe FIITJEE but I am not sure) to sign up for DLP but they were pretty professional, polite and non-pushing unlike Akash and didn't bother me again once I explained I already had joined a correspondence.
2. This was a rather curious incident. While in class 12th, I was once approached by a self-proclaimed 'agent' who offered me about 8-10 lakhs to sign up for a few institutes (plural !!). I didn't really talk to him much (and it might have been a fraud anyways) but in hindsight, I am curious as to how it would have worked!

I just wanted to stress that many coaching institutes very actively try to get to the toppers of some reputed coaching classes even before JEE and atleast in Kota 4  years back, I observed this to be more prevelant than paying people after JEE to use their rank (after all its cheaper and you get a lot of signatures by 'gifting' people books to help their preparation).

Post JEE
After getting a decent JEE rank, the offers were pretty much what one would expect :

1. Calls from a couple of coaching institute (Akash again being one) to 'congratulate' me and mention (not so casually) that their institute had an 'award' for the JEE AIR 2 (some figures between 5-10 lakhs) and ask if I would be interested in receiving their 'award'.
2. Like Prudhvi mentions, some random coaching class wanted to call me to address the students and pay me some compensation for the troubles (and am pretty sure would have then flashed my name in their adverts) but given that I am a rather poor orator, I did decline, so never got to see the end of that !

Another Student says

YES, It happens :D

There are some examples. 
1. I have earned a lot (more than 2 lakh) , I had got  rank between 500 to 1000 in General category .
2. One of my friend who has got 200-300 rank in JEE 2009. He earned more than 10 lakh only in Kanpur City.
Some of coaching are running only due to this idea.

I was in Kota for coaching. A guy from Times Education went from hostel to hostel for registering students for their correspondence course. Ok. Now the deal. Study material costed Rs. 250 and it entailed a "scholarship" of minimum Rs. 5000 for just clearing IITJEE with AIR(All India Rank) > 1000. Award amount was higher for better AIR. It was a win-win lottery for me as I was pretty much confident of clearing IITJEE.

A Kota Student says

I secured an AIR 31XX and guess what, they listed out my name on their website. They didn't even care to hand out the money. I had nothing to do productive during my vacations after my IITJEE, so I just bugged them with calls and mails for the money. I did get the money after 1 month.

P.S. After joining IIT, I came to know that many of my friends had their names on the website with the same trick. Howver, only few went through the ordeal of bugging them for the money.