18 dakara to Metoraremashitemo

Hieki Tsubasa wanted to rent an apartment of her own, but her savings weren't enough to cover the initial deposit. That was when her classmate Sagima Kakeru suddenly swooped in with a proposal for marriage. She immediately refused, but reconsidered when he offered her a million yen a month for the contract marriage…

(Source: Natalie.mu, translated)

Note: Chapter count includes the extra "Kimi ga Suki dakara".