Regarding your questions:
It costs 100,000 Gridcoins to create a poll, however these funds are not consumed at all during the poll creation or at its conclusion. The 100k poll creation limit serves to limit spam and congestion in the poll system.
The actual cost to create a poll is the same cost as sending a beacon, roughly 0.1 GRC or so, depending on data sizes involved in the poll.
Is there a way to add a "write your own response" choice for polls? Of course the creator would pay the data transmission fee.
At this time, no. The poll creator must specify all options that can be voted on.
Ok.