If the target creature is an illegal target by the time Brightmare's ability tries to resolve, the ability won't resolve
When you consider that rares and mythics are far more valuable than commons and uncommons are, which are pretty easy for everyone to access, in practice, this means that for every pack of The Brothers War you buy (assuming you eventually buy 10 to get the Golden Pack), you are essentially getting 1.6 packs worth of value, making Golden Packs perhaps the most generous improvement to the Arena economy ever
That being said, a single card being worth more than $1,000 is an incredibly valuable card
The Slasher deals another two, putting them to seven
2021-06-18 You are never forced to activate mana abilities to pay costs, so if there is a mandatory additional mana cost (such as from Thalia, Guardian of Thraben), you can decline to activate mana abilities to pay for it and hence fail to cast the suspended card, leaving it in exile
This allowed players like Orange an opportunity to try and break the Legacy metagame with cards that would essentially only be available for one week (the length of the Secret Lair preorder window)