To each their own. Though lets be honest, the quality of the expansion isn't going to change whether you or a million other people buy the emotes. It's just a cash-grab.
I like the idea of microtransactions for silly stuff like that, but they need to be priced a little more appropriately in the future. 10 dollars? Sure. 20 is really pushing it though.
As with everything, value is subjective. Absolutely nothing exists or will ever exist with intrinsic value - it will always boil down to aggregated individuals' preferences and willingness to trade off X for Y.
In this case, I'd wager they gave this a ton of thought. You don't just randomly price things - you have a vested interest in maximizing profit. With potentially millions on the line, you want to reach a number that satisfies
1) not leaving money on the table by selling below the point at which customers are willing to buy, and
2) not losing sales by pricing out your audience
Given the fact that, as a whole, this community is fairly polarized into ~casual and hyper dedicated players (1,000 hours in a year is ridiculous, seriously), they know there's a fairly substantial amount of people who are super committed and hardcore fans.
Pricing emotes at US$1.99, imagine they'd have sold 1,000,000 users the pack. Sweet, US$2m in the bank for a day's worth of animator salary.
Pricing emotes at US$19.99, imagine they sell this to only 200,000 - a fifth of the sales, twice the bank. The people who watch Bungie streams and read their blog and post theories on Reddit and get seriously riled up over minute tweaks to a virtual toy's performance. The kind - and I'm one of them - who would buy this regardless just because.
Voilà. There's no way to price discriminate perfectly (charging everyone as much as they're willing to pay). You need a blanket price that maxes profit. As a business, they don't have perfect information - maybe at US$10 they'd have made more money. Or maybe US$30 was that sweet spot. But they ran their numbers and this was the result.
Now the market determines if they played their cards right. Voluntary transactions take place when both parties agree they're getting something they value more than what they're giving up. In my case: I value the emotes and class items more than I'd value an extra US$20 in my account. Those who don't feel it's worth it just don't buy and are equally satisfied because they're keeping their US$20, which they value more.
Bungie is betting tons of people feel like me, but there's no losers when the stakes are determined by transparent, voluntary transactions between informed agents.