Cost of trickle down on components I enjoy buying used gear and building up bikes, but often wonder why some used/older components are only a fraction cheaper on sites such as eBay. For example, Shimano Ultegra 6600 10 speed shifters are currently going for anywhere around $100 - $150 USD when you could buy Tiagra 4700 for around the same price, which will most likely outperform the older series.

Do older bike component really hold value well or are older components expensive because of their, 'vintage' factor?
You've given an answer to yourself already - it's the 'vintage' factor. 

To elaborate a bit more on that have a look into supply and demand law.
For the items that are no longer in production the supply is limited and shrinking (not every component of a certain model will end up on the market, some get damaged, some age beyond the possibility of restoration, etc.). It's like with e.g. 10 y.o. whisky. They've put a certain, fixed amount of whisky into the casks 10 years ago and there won't be more.

When the component in question is not yet 'vintage' and not really wanted, the price drops. Once the demand is rising, so does the price.

Then you may encounter some components that were wanted from day one as they were not available on the amateurs' market in the beginning (vide some Campagnolo groupsets that were available only to professional teams).

And of course, the nowadays groupsets are often superior to their equivalents from previous eras (e.g. some say that a new 105 is at the level of 5 y.o. Ultegra, just to give you an idea) but it's up to you whether you want to ride something that everyone can buy off-the-shelve or rather some vintage and unique equipment.