It really depends on the startup, their traction, and your relationship with the founders. I've had folks ask for $10k-$20k for a seed round (the line between angel and seed is very blurry imho) with zero traction and a rough idea of the product market fit with a barely functioning MVP, and another team I spoke with wasn't talking to anyone without $100k to invest. Once you're past the seed stage, unless you're connected or an employee, it's $1MM+.
I focus on seed stage as I'm seeking high risk high return exposure for this part of my portfolio (and I'm not liquid enough to participate without investing in a fund for Series A on, unless I'm leveraging as part of a transaction).
As a founder, you don't want tons of small checks because of legal complications (> 99 entities on your cap table) and not a good ROI on your time initially + ongoing. A workaround is bringing in someone to run a syndicate on AngelList or some other bundling, making it their focus vs yours, and condensing to one entity.
The result is, as an individual investor, you can focus on those syndicates. Two benefits are you can spread risk by doing a wide variety of small investments, and while following more experienced individuals.
I've seen that changing a bit recently with the AngelList Roll-up vehicle[0].
It's almost like crowdfunding for seed rounds, but still with $5k - $20k kinda checks. But it helps out a lot with the legal side of things, so you don't need an investment agreement with a dozen different investors.
I focus on seed stage as I'm seeking high risk high return exposure for this part of my portfolio (and I'm not liquid enough to participate without investing in a fund for Series A on, unless I'm leveraging as part of a transaction).
(n=1, YMMV, accredited investor)