@Beregorn You're misunderstanding the concept. Kusunoki's future has changed after he met Miyagi. Now those thirty days are worth about 270,000 yen. He could have bought more lifespan in the past with just 30 yen but that was his previous, unfortunate future. This one is a future where he is destined for fame but he refused it. All to pay for Miyagi's debt which she inherited from her mother unfairly.
His life is no longer worth 30 yen a year like in his past but rather it has matured into almost 300,000 yen a month thanks to her.
Kusunoki doesn't care about money. He cares for neither fame nor historical immortality. He's simply fulfilled and satisfied to have lived two months alongside Miyagi as his observer and thankful for giving him a happiness which was impossible for him to achieve in his twenty years of life along with another, previously possible but not anymore, thirty years and three months of lost and unfruitful hope.
Nothing in this world matters anymore to him. He's thankful; not towards life but rather towards Miyagi's existence, her interactions with him.
Words can't really express how much I am attached to the main characters of this story. I really love this manga.