I don't think it's unreasonable to ask for contributions for a guild hall. Not all guilds will be capable of competing against the powerhouses that often dominate those Hall of Hero arena matches, so naturally the only resort for a casual playing guild is to buy a Celestial Sigil. If your guild leader somehow thinks that he will be the championing hero for the guild and win one, if you feel comfortable with this then go ahead and donate...
However, the contributions must be voluntary because you desire to help the guild you are in. If you feel that you are being pressured to donate, then I suggest you leave your guild and find another. I don't know about other guilds, but being in my guild means more than just wearing a cape (Which, btw, does not sport any flames and I paid for entirely myself at level 9.

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I spent the entire evening last night helping out a new guild mate get to Yak's Bend. I've seen the same areas of the map countless Times getting less experienced mates up to speed and helping them with small upgrades to survive battles. I do this because I enjoy getting people caught up. The faster they catch up, the more likely they will be contributing members of the guild. I received a nice 15-22 wingblade from a guildie who I initially helped out a lot but has since surpassed my highest level character. I did not ask him for this, he gave this up on his own accord. I am not planning on customizing it either, for I believe that some day I will hand this down to someone else.
So contribute to your guild if you feel good about it. If you don't, then I really suggest finding allegiance where you feel more comfortable.