I have been told a few times that in the event of selling one option against another held in a more favourable strike (as in your example) there may be no "exchange" requirement, but differing brokers may insist upon an "unposted commissions" requirement meaning that they either charge round turn on the opening of the spread OR charge a "maintenence" of approx the level of commissions plus the typical spread of the market eg. silver 8 ticks @$5 per tick plus two lots of commission of say, $40 meaning a total "margin" of $120 in this case.

The exchange margin requirement is zero in any case where you have a written leg further out of the money than the long leg - providing they are all exercisable into futures contracts and NOT the physical, in which case you'd be more likely charged the "commissions" margin as depicted in the above example. It costs a fair bit to Exchange for Physical, as anyone on here who held their long ag futures contract past "first notice day" may have found out to their cost....