about how much does mystical essence usually sell for?

According to current broker prices, I'd say about 750k to 1.5 million gold.

If it's a more popular sylesti (like ferrikkies for example) then it will probably sell for more, but it's kinda hard to find a person interested in buying mystical essences because of their expensive price (not everyone has a lot of gold and most prefer enhanced or regular).

The average seems to be around 1 million, but don't expect it to sell fast.

The problem with mystical essences is that they're technically worth a good amount but the market for them is so small that it's really hard to sell at 750k. The last two I sold were at 350-400k (nyte and vul I think) and even then, I had to relist them 4-6 times before they sold.

You night have better luck making a post and trading it/them for something you want in the Item Marketplace forum. That way, someone who doesn't have that much gold can still obtain one, and you can get pets/avi items/other things that you want or need.