While this jump may not sound staggering, the continued positive price momentum is indicative of this card's high demand
A "Gem Mint" 10 graded by trusted experts like PSA, BGS, or CGC can be worth 100x more than a "Near Mint" 7
Ideally, you would only use a 1-1-1 line of this Pokemon family, in a water type deck, you would keep this line on the bench, using Starmie's Ability, and then if the opponent had Pokemon BREAK in play you would move Starmie BREAK to the active Pokemon spot and use Break Star until all of those Pokemon are defeated, I believe all BREAK cards have less than 200 HP, so it should be a multiple knock out situation with Starmie BREAK's move
You can contrast modal cards with their opposite, punisher cards like Browbeat, Vexing Devil, and Risk Factor
For those Pikachu fans who have been priced out of the most expensive Pikachu cards like the Illustrator, some may have been driven to second or third tier Pikachu cards like this Holon Phantoms card