There is a dollar amount I would consider doing it for. $100,000 is a seriously low offer.
Most of these weird "would you do X for X dollars" are always weird things for weird amounts. Like asking a straight guy if he would suck a dick for $1,000,000. A lot will say no, and all of them are liars, and when really presented with the offer they would do it for far less. I guess the only examples I can think of are straight guys being asked to do gay things for money, since I've had gay friends ask me. And I do the same thing, I propose that it has to be some ridiculously high amount of money, but in reality I think I would cross that bridge for not all that much.
But $100,000 isn't really that much money in the long run. The average person makes about $10,000-$20,000 more in two years, at least in the US. If your dick is big enough, and you're willing to do the paperwork, you can go on Chaturbate and start making extra money with your dick. It won't be $100,000, but it will be more fun and could lead to something more lucrative if you're willing. I bet you could work your normal job, and make an extra $10,000-$20,000 in a year if you spend a lot of time on it and are a good entertainer.
For $100,000 I could take time off work without touching my savings. No, I wouldn't reduce the size of my penis for that. For $1,000,000, I could like the rest of my life without working if I significantly reduce my standard of living. Since I have a job that will do more for me, and I like my cock, no, not worth it. At $10,000,000 I would start considering it seriously and by $20,000,000 I would probably do it. If I'm going to give up part of what I like about my cock I'll need to increase my standard of living significantly, be able to retire today, and have some of the other benefits of a large sum of money to make that sacrifice. Still, any kind of body modification, especially for a deficit, I think I'd be less likely to do that regardless of the money.
I contradicted myself, but I always do that with these types of questions. I think they are fun thought experiments.