Just from collecting PLAY promos and searching for Vaporeon, Jolteon and Flareon on a near daily basis I definitely feel like there is a stronger drop off from Jolteon to Flareon (compared to Vaporeon > Jolteon).
Vaporeon and Jolteon feel reasonably available on the Japanese marketplaces, but Flareon copies show themselves about as often as Espeons, maybe a tad more.
For everyones convience here the total PSA population of the goldstars:
Points required:
Vaporeon 10,000
Jolteon 20,000
Flareon 30,000
Espeon 40,000
Umbreon 70,000
Total graded (by PSA) population ‘corrected’ for points:
Vaporeon 241
Jolteon 356
Flareon 291
Espeon 236
Umbreon 252
Jolteon definitely seems overrepresented, this could be due to the 20,000 point mark also being the threshold for ex member status, which gave some additional benefits. I think based on the point system casual but diligent players could attain Jolteon without much trouble, in a reasonable timeframe (1 year of weekly attendence of events). This is about where I expect the dropoff to increase rapidly.
We also have to keep in mind that as said above, the higher tier cards were awarded to players who were more likely to look after them, and the higher prices means there is more incentive to grade than the lower tiered cards, and/or more likely to get them into circulation.
So my best guess (and it is a complete shot in the dark):
4000 Vaporeon (6,0% graded)
2500 Jolteon (7,1% graded)
1000 Flareon (9,7% graded)
450 Espeon (13,1% graded)
250 Umbreon (14,4% graded)