Ahh I never even considered the cost of implementing artwork. Honestly if I were you, I'd start charging for the game with or without artwork - or perhaps make a patreon. I would certainly pay for a game like this.
A patreon is something I'm eternally considering on-and-off. Charging for MERCANOID now would leave a sour taste in my mouth, but not all future projects will be free, no. As my quality grows, I'll be more confident in charging for a game, allowing these older titles (and occasional free smaller projects) serve as my demos.