By my calculations, Park is worth 8.6 runs more than Franco in the field. I have Park at 0.3 runs above average (average for my world) and Franco at -8.3, and that doesn't even take into account double plays. That's for only half of the season.
I think Park is underperforming a little bit as he is making more errors than I would expect, but Franco is doing pretty terrible.
If you want to do a really rough calculation, estimate the value of an additional baserunner at half of a run. So Park has 3 more errors, but Franco has 15 more minus plays, which means that Franco has allowed 12 more baserunners. Add the 4 baserunners that Park "robbed" of hits with his plus plays and you have a difference of 16 baserunners or about 8 runs. You can get a little more accurate by equalizing for number of chances and some other minor stuff, but that gets you in the neighborhood. 8 runs in half a season is pretty significant, not to mention the echo effect that more baserunners means higher pitch counts, which means starter fatigue, etc.
Additionally, I perceive a (-) play as worse than an error because, by my understanding, a (-) play is a ball that would almost always result in an out and thus it is always adding an additional baserunner. In contrast, an error can be a ball where the runner would have been safe anyway, but he is able to advance to second (or whatever) on an errant throw. It varies from situation to situation, but, on average, the cost of a baserunner(s) advancing is slightly less than the cost of an additional baserunner.
So bottom line, you would expect Franco to be terrible in the field and he is. You would expect Park to be a bit above average and right now he's at about that or slightly underperforming.