Are there less expensive alternatives to Apple-branded Mini DisplayPort adapters?

I'm with you. I don't know why Apple has such a high mark-up for a simple adapter. But you can get them for substantially cheaper through aftermarket, 3rd party sellers.

I purchased a Mini DisplayPort to DVI through Monoprice for $6.65 USD while Apple wanted $99 USD for theirs (which strangely came with a USB "extender" I had no use for). I thought it was a typo!

Here's a full list of available Mini DisplayPort adapters from Monoprice.

They also provided free shipping and the while the quality of the adapter is not the same as apple's is it quite decent (reminiscent of a standard computer peripheral component from the 90's). But most importantly, it works just fine!

There is a (lot of) confusion on this topic despite it's laden clear in Wikipedia. DisplayPort is a totally different standard than DVI/HDMI. However, most ports (inc. Apple) are DisplayPort++ which means they are capable of using the same pins to deliver a DVI signal so you just need a DisplayPort and a DVI connector and solder wires between the pins. This results in the so-called single link DVI -- fine for most purposes except 30" DVI monitors. If you have a 30" monitor with a double link DVI input you need complex electronics that take a proper DisplayPort signal and converts it to a DVI signal. This requires power which is usually taken from an USB port. And it's every expensive, indeed.

Yes you can.
You have a lot of them on Amazon and also some 3rd party adapters on the apple store itself.