I don't know. But look at the ASM #100 cover commission from 1994 Romita did. It sold in HA for $77,675 (link below). Who's to say what an unused cover or a commission cover redue will go for compared to one another.
https://comics.ha.com/itm/original-comic-art/john-romita-sr-the-amazing-spider-man-100-cover-re-creation-original-art-1994-/a/7036-92223.s?ic4=ListView-ShortDescription-071515
Also, if you look on HA at the Batman #210 unused cover, it went for $3734 in 2014. That's about what I'd expect a vintage commissioned cover redue to sell for.
The used cover (different artist) went for $30,070, also what I'd expect in comparison.