you give money for expected performance, not past. Harris, due to circumstances outside of his control, will very likely not repeat his contributions in fielding from last year. and he had a career year with the bat too, so you'd be making a bad gamble that he'd repeat it. but i don't think it's ridiculous to say his contributions on the field were of significant value last year. you can use this information to make a good projection, using regression and three years of data. but if players had arbitration which paid them after the season in relation to their on-field performance (on a FA payscale), fangraphs' salary value would provide the most accuracy of widely available statistics.