Prestige awarded seems to be set to a low tier level, it's still basing it on your normal Tier. I have personally gotten 300 prestige on Tier 1 or lower characters.
Last year when I ran it on my end game or high level alts, I only got 1 prestige. On my lower 30's I received 300 or less depending on their level (just like alerts) and my sub 30's (which I only ventured a few in there with friends or league mates) got the full 300. So it appears they are doing the same thing since they added it again.
I've been getting 300 for other Seasonals on all alts. The fact that we are Clamped to sub 30's means that we should get rewarded as such. Otherwise remove the clamps.
I agree with you that we all should be getting 300 if we are clamped. I think I had one alt get 75 today and one get 12. The rest that I did the event on all all got 1.