MacStadium's prices are "rather less than 20%" of AWS's prices. To make sense, AWS would have to be comparable to MacStadium's pricing.
And don't forget the 24-hour minimum. If your duty cycle for a Mac Mini is only 20% per day every weekday, well, you can't rent a Mac Mini for less than a 24 hour period, so instead of paying for 20% of a month, you'll be paying for 20 work days, 60% of a month. On AWS, that would run you $546/month, 4x as much as you'd pay MacStadium for the entire month.
AWS's price is only comparable to MacStadium if you need five 24-hour periods per month (or less).
And, sure, AWS's prices will decline at some point, but I'm not expecting an 80% price drop this year.