Airport Express tickets and Octopus cards are different things. Octopus cards are cash in HKD only.

Minimum HK$150 to start a card, with $100 spendable dollars and $41 of deposit that will be refunded (along with unspent credit) to the OP. [$50 if over 3 months after initial purchase.]

So, the OP needs to get minimum of HK$150 in cash to get the Octopus. If he only uses it for the two $24 bus rides, he'll get $52 + $41 = $93 to spend at the airport.

One other thing, the bus fare going back to the airport on E22A is $24. He can actually spend the Octopus card down to as little as $0.1 left before his bus ride, and can still get on. He'll then have a -$23.9 remaining value. Not good for any further purchase, but this way, he'll get $41 - $23.9 = $17.1 back when he returns his Octopus at airport.