The strike price is defined the products are being created.
For products created by Opium.Team, we analyze the tickers before deploying them to make it a nice opportunity for sellers and buyer.
What you pay for the contact is up to market and your limit order. Market expectations are represented in "order book" tab.
It's a total margin required for derivative settlement.
Example you can see here:
If you are trying to sell something for more than collateral, nobody should buy it. However, non-custodial platform does not restrict you from that. Somebody can ask a million for writing that option and that will go to order book equally with other orders.
You can set manually the time of your order expiration. Expiration of the order is the time until it's matched.
After chosen order expiration it will be removed from order book. See video below.
So you can create order for a few minutes. hours or days or just without expiration "good till cancel"