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!