If I were getting a 50% equity stake, I wouldn't expect a salary. That's basically a cofounder role, and the risk/reward work out appropriately. I'd expect to live off my own savings - but I'd also expect significant input into the business, including the ability to change its direction if the original idea really wasn't working. Basically, if you give up 50% of the company, you're in this together, better act like it.
I might expect a relatively small (cover living expenses) salary if I were taking on, say, a 20-30% equity stake as a cofounder in a pre-traction startup. This is sort of a no-man's-land of risk/reward tradeoff, though. There's an awkward dynamic when you're paying someone else's living expenses and yet their salary is still well below market-rate. It's worked for a couple startups (Hotmail was one, Intuit may've been another, though the equity stakes for Intuit all got renegotiated when they almost went bankrupt), but you don't see it often.
Once you're down in single-digit equity stakes, it's expected that you'll pony up market-rate salaries. That's what fundraising is for. :-) The equity stake for early employees is there to compensate (and incentivize) them for the possibility that their salary may not be their tomorrow, not for the lack of a salary.
True, although if I were coming on-board as a cofounder, I'd expect to be there for the long haul, so they don't affect me all that much. I'd prefer to see both partners have pretty standard vesting, probably 4-year.
I do take vesting into account if I'm joining as an early employee and I expect the company to be bought out soon. I believe that acquirers can do whatever they want with an employee's unvested options, and this is a pretty easy way to get screwed.
The minimum and optimal is enough to keep them break even financially, nothing more. That could be $40k/yr or $90k/yr. Totally depends on their specific situation.
Whoever pays that salary should get equity in exchange, whether it's you or an outside investor. Ideally it's an outside investor (who can afford to lose it). Either way it should be at a reasonable valuation.
I would raise initial salary from my own pocket for the lead CTO, so I'm curious if I'd have enough. $90k+ is not feasible.