I'm faced with cleaning up our entire fleet of of police bikes. They were all stored outside for some time while our new storage shed was being built, and in the process of moving them I noticed that they were all really cruddy.
I need something I can buy in bulk; little bottles of boutique degreasers are nice but expensive. I've used simple green in the past; are those big jugs of orange-based stuff they sell in the big-box stores any good?

kerosene, just watch the plastic bits.

Those big jugs of citrus degreaser are pretty much the same thing as Simple Green and should work just as well.

Are the bikes really greasy or just splashed and dirty? If it's mostly dirt, dish soap in a bucket of water will be just as effective.

There's plenty of just plain dirt; at some point they had the garage where the outside bike rack was steam cleaned...Threw all manner of crud.
However, the drive trains are all gunked up as well.

Kerosene/petroleum based fuel of your choice (Kerosene is the least likely to explode and/ or make you high) with good (go outside) ventilation. Citrus degreaser also works reasonably well if you add a bit of water to it, is friendly on plastic, and doesn't have the same fumes. However, if it says to add water, do. If you don't add enough water in my experience it actually doesn't work as well.

I use kerosene to clean individual parts after they have been removed from the frame but would never use it to clean an intact bike. I use it in a plastic pan with a brush and then discard the dirty stuff with recycle oil. I would not want to have it get all over the ground as it will if you try to clean an entire bike with it.

Your best bet for large jobs is to contact Safety Kleen. The small parts washer is only $270. If your department has an auto shop, I bet they already have a system and a contract for the solvent.

For a fleet, I would use paint thinner or the cheapest equivalent and a large bucket or two. Remove the offending components, and sit outside with a brush. If they're really bad, use the first bucket for primary removal of most grime, and the second bucket for a "rinse" with clean solvent. For chains, a plastic 2-3 liter soda bottle and a minute of shaking followed by a hot water rinse and a blast from an air compressor.