That's an easy one...
$500 additional is actually very reasonable considering the amount of extra materials that go into them. Monoblocks amps double the following components... chassis, transformer, PS capacitors (not the BG add-on), rectifier, ultrawire, and the cost to have the chassis laser cut and engraved. All of these items are expensive, because I use extremely high quality parts. There's no skimping anywhere.
If you're trying to decide which version to buy, I would recommend the monos only for the following reasons:
- sytem layout: my customer keeps his amps near his speakers, and didn't want to rearrange his whole system and purchase all new cables to accomodate a stereo amp.
- extra RMS power: The monos will 500W RMS into 4 ohms, whereas the stereo amp will have 250W into 4 ohms. Note, this is different from instantaneous power, where both amps will do 500W into 4 ohms. Most people will never notice the difference, unless you have VERY inefficient speakers that could require up to 500W RMS power.
- sound quality: I am expecting the Monos to be slightly better in sound quality, but not much. We'll be comparing the two versions on very revealing speakers... VMPS RM40 w/ TRT. I would expect that purchasing the other upgrades to the stereo amp would be the smart way to go if monos aren't necessary for the other reasons.
- home theater/multichannel: if you want five or six symmetrical amps for a HT system, monos are the way to go

I will also add that introductory prices are going up soon, so let me know if you are seriously considering buying an amp.