How much can a BTech + IIM A/B/C (1 yoe as an engineer) expect to make after graduating from IIM?

Median salary is what you should expect.
Placements in IIM depend on a lot of arbitrary factors that are out of your control. Things like your 10th, 12th and UG CGPA can be used by your prospective employer for shortlisting.

A particular company that is a high payer might nor turn up that year cause they had a tiff with placecom, or they can turn up and decide to hire for diversity and hence you might not get shortlisted.

You might end up in a GD group with anxious peers who can reduce the entire GD into a shouting match (I experienced this twice, in the same day).
You might have been rejected 3-4 times and your confidence is low and you still have to attend 3-4 more interviews for which you arent mentally prepared and hence flunk them all (happened to a very close friend of mine - a super interesting guy who crashed out of 8 interviews in a day and just didn’t recover well until day 2.