Well, I've been working with Unity for 11 years now, but my experience with building for Linux is limited, so I'll go with experience from mobile and console development, and say that with shaders (which never ceases to surprise with platform-specific glitches), filesystem (although Unity is supposed to abstract it out, once you get serious about resource management, this abstraction starts to leak), input (because you want to support gamepad with minimum hassle for the player), and million of unknown unknowns that hopefully will be found by QA, but realistically, by very angry players, I'd budget at least a couple of man-months for this. With sales in hundreds or even tens of thousands, we can expect to get about 2% of audience from Linux, which will account for single-digit thousands, or hundreds of copies sold. With a price tag of $10-20, after Steam cut, taxes, regional prices, sales and refunds, it's about $5-10 of revenue per unit, so in total, about $10k.

If that would just be the money a single developer got for 2-3 months of work, it could still be fine for some regions (not everyone on HN is from US). But with a studio, you also have to spend money on marketing, payroll taxes, rent, and development of future projects, which most likely sell even worse.

So, no, I really don't see how porting to Linux would pay. Most of developers that I talked to who did it admitted that supporting OSX and Linus turned out to be a giant waste.