Great looking boat!
I just helped a friend sell a similar Mako 21 for $14k, and his had an older Johnson 2S on the back.
Based on that, I would guess $18k, depending on location, but you are best to search online to see what's out there. That will tell you market value.
Your boat looks very clean, and looks sell, so it may surprise you! Boats are running high in the mid-Atlantic area right now. I've sold two project boats in May because of offers I couldn't refuse. Then I searched and saw what they are selling for, and am kicking myself. But I'm happy to have them gone, and they got a great deal, so all is well.
Best wishes for a quick, full value sale.