Unless their policy has changed dramatically, HP uses proprietary, half-sized (at least for the mini-towers) power supplies that can only be replaced through them. Good luck getting one for $30. Maybe for an HP mid-tower case, a standard power supply might physically fit, or be made to fit, then all you would have to do would be to make sure there are no proprietary connectors on HP's power supply.
This discussion is pointless. I was just trying to caution Muskrat about potential problems upgrading HP systems (which I know from personal experience).