A flat payment of even $8000 is not reasonable for someone to move across the country, even if they are moving alone and seeking small, affordable apartments.
A minimum relocation package should include having the company handle the arrangements for a full-service moving company who will pack your things in boxes and unpack them in the new place, flight and hotel for 1 or 2 visits to the new city for housing search, cover all up-front costs with renting a new place (fees, deposit, first month rent), and still leave you with a few thousand dollars on top of that for incidental expenses, unplanned costs after you've moved in, etc. And if they need you to relocate from an area with accessible public transit to an area where a car is required, then a sign-on bonus that covers most or all of the cost of a reasonable quality car should also be standard.
This is not luxury stuff, this is just the basic human cost of moving across the country. If you take the risk of a new job in SF and move all the way there and it doesn't work out, well you don't get a refund on your moving expenses and you don't get compensation for any personal time you took (e.g. if you moved yourself with a UHaul).
And what if you injure your back while carrying furniture down the stairs? Will your new company cover the medical bills? What if the moving company you hired damages your stuff or loses it? Will the new company reimburse you?
This is why it's so necessary that the company assumes the risk of these things. The company is funded and/or profitable (or else shouldn't be considering hiring someone who requires relocation). They can afford to bear the relocation costs and the risks. A lone individual may or may not be able to, and can't risk their livelihood on their ability to handle the relocation risks.
But it's all too common that a start-up offers $2500 to a recent college grad who has been eating ramen for the last week. It sounds great, and they don't know enough about career status and so forth to realize that if you admit you are willing to physically haul your own stuff around, it already paints you as a low-status subordinate. These folks unblinkingly accept precarious and unreasonable relocation scenarios, and then the company can hide behind a claim of "well, no one else complained about our relocation assistance" when negotiating.