It's highly recommended to NEVER go for a black just because they're on spotlight unless you're willing to burn over $200 (300 SC). On AVERAGE you get a black every 33 summons (165 SC) but there's no guarantee it's the one you want, and that's IF AND ONLY IF you actually get one. I have gone through 250 SC (50 summons) in 2 days and scored 0 blacks (and only 2 plats when average says 5) so do not EVER think spotlight means there's a good chance of getting a black unit. Spotlight does not change rarity chance, only chance within the rarity.
Rarity Chances:
Black: 3% (1 in 33.3, 166.5 SC per)
Platinum: 10% (1 in 10, 50 SC per)
Gold: 37% (1 in 2.7, 13.5 SC per)
Silver: 50% (1 in 2, 10 SC per)
Spotlight basically doubles, perhaps triples, the chance of the given unit being drawn IF you hit that rarity. Not in game, but for the sake of example, let's say there's 10 blacks in the game (obviously there's more than that)
Normally, IF you get a black, 1/10 chance of it being a specific one
Highlight means 2/11 = 1/5.5 or maybe 3/12 = 1/4 chance
BUT!!!
.25 (1/4) * .03 (chance of black at all) = .0075 = .75% chance = 666.67 SC per to get THAT ONE black if there were only 10 blacks in the game and highlight triples chance within rarity