Summary first

; anyone interested can read on for more:
For me, the current entry fees Euk has listed at 50g for Sealed and 3 serie MC, and 35g for 1 serie MC, are all ok.
AC fees are too high; I would not pay 50/40g respectively to enter 3/1 serie AC league.
I probably would pay 35g (or maybe 40) for 3 serie AC, and 25g for 1 serie AC.
In more detail

:
First, a couple of points I think are relevant to bear in mind on this issue:
1. It is not valid to look at entry fee in isolation from prize support. The two are connected as part of a risk/reward balance.
2. Entry fee/prize support, ie gold, is of relevance (and probably of interest) only to players who want to collect My Cards (for whatever reason).
I am such a player. Although my impression is that there are a large number, probably the majority, of players who do not ever play with MC. So concern about fees and prizes may be something of a minority interest pastime.
My own view on entry fees is subjective, affected by my self perception as a player.
My experience to date suggests that I can reasonably expect to win about half my league games. This seems likely to place me in the 33rd-64th group in Sealed, 9th to 16th in MC or AC. Of course I might do better, but then I might do worse.
Buying packs is risky as we all know.
Agreed. Actually it is more than risky - would I spend 10g to buy a pack, or to buy that
Aragorn's Bow that I need? I have never bought/will never buy a pack.
The contents of those packs is irrelevant.
I do not share this view. The contents of a pack are the equivalent of 4.16 gold. Occasionally a pack will provide a R that I need (or even a foil R), and for sure provides a chance of that, but generally this does not happen. A prize pack does not have a value of 10g.
So, from my
perspective (throughout I assume a target of achieving 5-5 per serie), where does the risk/reward balance lie?
a) 50g for 3 serie MC is fine. 50g fee for a prospect of winning 14 packs and 6 C of my choice; equivalent to 60g and 14 chances to get a useful R.
b) It follows that 50g entry fee for Sealed is also fine by me.
c) 1 serie MC - 35g for potential 8 packs and 2 C, equivalent to 34g and 8 R chances. Also good. Lower potential reward ratio, but the investment of time and effort required is less than in 3 serie MC.
d) 3 serie AC ratio is 50g fee for 48g in cards (11 packs) and 1 serie AC is 40g for 21g (5 packs). On that basis, 1 series AC at 40g is way too expensive for my liking. In addition, AC is less fun. Call me sentimental, but I have greater attachment to decks made from cards I have individually selected, whether the deck is "tier 1" or not. Anyway, at times I like to play eccentric decks. There is more scope for this in MC.
We WANT you to have patience regarding building your collections.
Absolutely fine by me. In fact I prefer it that way to how it was before the January 2014 reset.