I was concerned about that, but I was surprised to find that the power consumption and even more importantly, the noise level of some of the drives is not too bad. There are two that I'm using, one is a Fujitsu 15K RPM drive which is a 3.5" form factor with 2.5" platters. It has fluid bearings and is very quiet and works fine inside my SE/30. The other which I'm going to swap in once I get the mounting adapter is a Seagate Savvio 2.5" U320 SCSI drive. This one is nice in that it's a 10K drive with fluid bearings and is even quieter than the Fujitsu, both being much quieter than the stock drives these old machines came with. Another perk is that it supports termination so with the $2.75 adapter you can plug it straight in without using a terminated cable. Power consumption is about on par with the old 3.5" drives that originally came in these things. The Savvio drives come in 36GB and 74GB sizes, both ridiculously large. Since partitions can only be up to 2GB most of my drive is wasted space but it's a cheap and relatively modern storage solution. As long as I can find Savvio drives for <$20 I'm not going to bother looking for affordable SSD options. If you mount them with rubber grommets to isolate the vibration, you can't even hear them over the fan.
Avoid the 3.5" 10K RPM drives though, most of them have conventional bearings and are very noisy. I have a few Seagate Cheetah 10K drives that sound like a dentist drill and they're bulky things that draw a lot of power too.