I mean, Martial is only in his second season now, he didn't move to United ages ago so I'm not sure if we can talk about "today's market" as if his transfer was in a different era. I'm not sure the market has changed all that much since then, but I'm not basing that on any sort of analysis of transfer fees so perhaps they have significantly inflated in the past year.
Yeah it's not too crazy to imagine, if it was my tone towards Maverick would have been much more incredulous. As it is I think I'd stick with saying that I think 100m is a stretch, with the proviso that I haven't seen the kid play aside from a few clips of goals. I think he'd have to be "next Messi/whatever" levels to reach such a fee.
Also as you said Martial was 60m if you include the add ons, I'm not sure what the add ons are but there's a chance Man U might not ever end up paying some of those clauses. I think the Martial upfront fee was £36/38m (or something close to that) which definitely makes £100m feel like more of a stretch to me.