Ask HN: Equity for Short-Term Contract?

Recently I have been in conversations with a founder who has some product developed, but is looking for significant additional development. Initially they were looking for a cofounder, but I think now they kind of are in the position where they want to hold out for a "unicorn" cofounder to meet some of their very specific needs.

I mean, yes: many red flags with that alone, but wanted to give some background.

They approached me with a short-term contract (less than a year) to do significant product development - essentially take their existing small product (which probably has 20-40 man hours of work into it, it's basically just a site with a login) and bring it to a point where they can generate revenue.

Ok, fine - but how things are evolving, it sounds like this is becoming a fixed cost contract that is paid solely in equity.

Does anyone have any experience with this? It seems like a strange arrangement, as there's a conflict with requesting what I would feel would be a fair amount of equity (which would be a significant amount) but since it is short-term, it also seems unfair to request such a large chunk of equity for such a short work period. The traditional model makes sense here: long term relationship with a large chunk of equity vesting over a few years, but trying to wrap my head around what a "fair" amount of equity would be for a short-term contract.